add an option to keep sub directories under @asset_paths

For example, if your asset path is 'assets/javascripts' and you have a
coffeescript file 'assets/javascripts/foo/baz.js.coffee', the output file
will be 'foo/baz.js' underneath the destination directory.

Previously, the plugin would just create the file 'baz.js' under the destination
directory.
update the README to document :keep_paths
Merge pull request #4 from caleb/master

Add an option to put compiled assets in their sub directories if they have any