Register as preprocessor instead of engine

This means we no longer need to use awkward filenames like
file-rtl.css.rtl for our RTLization targets and can use the more
sensible naming convention: file.rtl.css
Merge with master
Fixed compatibility with Rails 3

- Improved Railtie initialization
- Renamed RtlTemplate to RtlProcessor
- Some other minor improvements
Removed dependency on Tilt
Don't require jquery & jquery_ujs in dummy application.js
Single line config for disabling asset cache store in Rails 4
Move all requires to rtlize.rb
Make it configurable & improve defaults

* Added config option for rtl selector & used [dir=rtl] as default
* Added configurable rtl_locales option.
* Added a helper `Rtlize.dir` that outputs direction (ltr/rtl) for
  specified locale (or current locale)
Register as a postprocessor not as a preprocessor
Fixed a bug with transforming multiline CSS
Bug fix: Respect no-rtl markers even if first selector after them is ignored.

Also did some refactoring.
Added a test case for handling of @media queries
Removed unreachable code & added more tests
Use plain old ruby instead of mattr_accessor
Transform properties with IE hacks properly
Minor modifications
Use 4.0.0 as rails version by default & add debugger
Add support for backwards compatible syntax for rect shape values
Added more tests
Use \s not \W for matching whitespace characters