Fix for path resolution.  Add options for including specific extensions.  Add some documentation.

* Resolve path names of 'rfolder'ed files relative to the file which 'rfolder's them.  Handy when you're running browserify from a different directory than your source.
* Allow user to override the list of extensions that will be required via 'options.extensions'.
* Allow selective keeping of extensions via 'options.keepExt'.