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

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. This would cause clashes if two asset files had the same base name.

I've made this an option (keep_paths), and set the previous behavior as the default (keep_paths=false), but unless I'm missing something, creating these directories should be the default behavior (or the only behavior).