Added support for LESS dynamic CSS

All tests pass, and I added a few more tests for the CLI to verify that we can correctly parse LESS and SASS files as well as CSS.  The SASS test stylesheet specifically covers the shorthand format.  I also did some minor refactoring on cli.rb to make it easier to test, and bring down the complexity of the execute method. 

LESS is an optional dependency, so users don't need to install the LESS gem if they're not going to use it. LESS tests are skipped if the LESS gem is not installed.

I merged all the latest changes from master before this pull request, so all the refactored stuff and bug fixes are already in.
 
cheers