Update README

* Remove references to pre-roadie code; it has all been rewritten by
  now.
* Talk about "pre-release" instead of "experimental"
* Add some more links to help people get started.
Remove (the pending) Rails integration tests

They are to be moved to roadie-rails.
Remove ActionMailerExtensions and Railtie
Remove Roadie module methods
Move lib specs into spec/lib/roadie
Remove unused mid-refactoring-step code
Refactor and improve readability of Inliner code
Asset providers should return Stylesheets instead of Strings

Also extracted the shared examples into "lib/roadie/rspec" so users can
include them too and verify their custom providers.
Document the built-in provider examples in the README
AssetScanner returns Stylesheets instead of Strings
Finally send Stylesheets to Inliner instead of CSS strings

This should make it easier to trace errors as well. We can refer to the
stylesheet name on exceptions.
Wrap Inliner specs in Roadie module to cut off namespaces
Output stylesheet name in error message
Add integration specs that show Roadie's functionality
Oops. NullProvider were still providing strings instead of Stylesheets
Make Roadie crash on unknown assets
Small adjustments to README and integration_spec