1. Clean up screenshot helper

Updates documentation to be clearer and separates the concerns of saving
the image, setting the image path, and displaying the image.

2. Remove Rails provided assertions for selectors

This was moved upstream to Capybara and is no longer necessary to be
included in Rails

3. Remove form helper

The form helper is pretty specific to Basecamp's needs and may not be
helpful outside of Rails.

Since I've moved the teardown code that contains the screenshot handling
to be generated when the application is generated this code was
interfering with the screenshot taking.

Because this runs before any app teardown code we would be resetting
sessions before taking the screenshot, resulting in a blank browser
window. The code to reset the sessions must come AFTER a screenshot has
been taken.

If this is not set Webrick will log **everything** to STDOUT and
distract from the running tests. This will instead log to the log file.
This example was extracted from the Capybara source code.

Call doesn't make as much sense here, we're really starting to run the
driver.

Many changes have been made since the beginning so documentation needed
a refresher.

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.