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.

Rubocop / code climate don't like single quotes and prefer doubles.

Puma is the default webserver of Rails. Because of this it doesn't make
sense to run tests in Webkit if the default server is Puma.

Here I've refactored the webserver to be it's own standalone module so
it can be shared between Rails' selenium default driver and Capybara's
defaut drivers.