performance improvements/optimizations

- removes dependencies on sass, coffeescript, and compressors (still
  used in testing but loaded via Gemfile and not gemspec)
- adds asset_cache config key
- when asset_cache is strue, will return sprockets index instead of the
  raw environment
- sprockets environment is no longer a singleton
- only sets compressors in rake task
- since middleware is no longer used automatically by the Sinatra
  extension, needed to add it to the sinatra extension specs to make
  them work again
- renamed the rake task class to follow file name to be: ClassyAssets::Tasks
Bump to 0.9.0