allow full paths to be added to sprockets load path

- ClassyAssets::Configuration.asset_dirs is now
  ClassyAssets::Configuration.asset_paths which now takes full paths in
  order to support adding assets from outside of your project (e.g.
  from a gem)
- Use ClassyAssets::Configuration.build_asset_path to pass in folder
  names (or path segments) relative to your projects asset to have their
  full paths created for you. Takes an array and returns an array. You
  will still need to add the results of this message to the asset_paths
- asset_paths is still loaded with common defaults relative to your
  project's assets directory
Bump to 0.1.0