This is a major rewrite of what existed previously. After discussing
this feature with DHH I realized that I was looking at the setup all
wrong.

I had originally mentally broken it into "what Rails wants" and "what
Capybara already has".

What happened after looking at it from DHH's angle was that I saw there
was no reason to group settings by Driver but instead the following
groups:

- There will always be a `Driver`
  - This can selenium, poltergeist, or capybara webkit. Capybara already
    provides all of these and there's no reason to break them into a
    category of Rails' usese Selenium like this and Capybara uses it
    like that.
- Only Selenium drivers care about `Browser`
  - Because of this it was weird to set it only in the Rails end.
  - Therefore only `Browser`, and not `Driver` cares about
    `screen_size`.
- Puma is the default `Server` in Rails
  - Therefore there's no reason to explictly support Webkit

Once I looked at it from this angle I was able to abstract all the
settings away from grouping the drivers with their options.

Now all the driver, server, and browser settings are abstracted away and
not part of the public facing API.

This means there's no requirement to initialize new classes to change
the default settings and the public options API is much smaller.

All of Rails preferred defaults are still there (selenium with port
21800 using the chrome browser with a screen size of 1400x1400) but
changing these no longer requires initializing a new class or
understanding which driver you're using underneath (rails defaults or
capybaras defaults respectively). Rails opinions are now simple defaults
instead of doing a them versus us setup with Drivers and explicit
options.

Changing the defaults is simple. Call `driven_by` with different
settings to change the defaults which will on their own initialize new
classes and change the default settings.

Use poltergeist with port 3000 for Puma

```
driven_by :poltergeist, on: 3000
```

Use selenium with the Chrome browser and a screen size of 800x800
```
driven_by :selenium, using: :firefox, screen_size: [ 800, 800 ]
```

The entire setup of how browser and drivers interact with each other are
abstracted away and the only required argument is the driver name.

Originally I had set up system testing to have one configuration option
to be set in the test environment. After thinking it over I think a
generated class on app creation would be best. The reason for this is
Capybara has a ton of configuration options that I'm sure some folks
want to use.

Thinking about how we handle screenshots, database transactions, and a
whole bunch of other settings it would be better for users to be able to
turn all of that on and off.

When an app or scaffold is generated a `test/system_test_helper.rb` test
helper will be generated as well. This will contain the class for tests
to inherit from `ActionSystemTestCase` which will inherit from
`ActionSystemTest::Base`. Here is where users can change the test
driver, remove the screenshot helper, and add their additional Capybara
configuration.

By moving to the TestUnit Railtie, and doing the file requirement
inside the onload call we can avoid loading ActionSystemTest in
production and load it in the test env.

This is important for performance reasons - loading up unnecessary files
and object is expensive, especially when they should never be used in
production.

Renames `Rails::SystemTestCase` to `ActionSystemTest` and moves it to a
gem under the Rails name.

We need to name the class `ActionSystemTestCase` because the gem expects
a module but tests themselves expect a class.

Adds MIT-LICENSE, CHANGELOG, and README for the future.