removes obsolte class
major rearchitecture of the entire library

- obsoltes and removes ClassyAssets::Configuration, use
  ClassyAssets.config instead
- implements ClassyAssets::Config as a singleton class, which should be
  accessed through ClassyAssets.config
- extracts sprockets environment from configurtion class and implements
  it as a singleton class in ClassyAssets::Sprockets, which should be
  accessed through ClassyAssets.sprockets
- rewrites the Rack middleware to make sprockets to most of the work
- throws ClassyAssets::Errors::NilAssetRoot exception if asset_root is
  not configured (no longer has default option)
- many config options have been renamed; valid options are now: `:asset_debug, :asset_digest, :asset_host, :asset_paths, :asset_prefix, :asset_root`
- public_path option has been removed (it was unused anyway)
- specs have been cleaned up and sped up, only a few known areas are
  untested at this point
Bump to 0.6.0