[WIP] Add WADL support

This PR adds a [Web Application Description Language](http://www.w3.org/Submission/wadl/) formatter. It's the only language that describes REST services like WSDL for SOAP web services.

It becomes really useful when you want to generate clients, it's a common practice in Java world, and I think it can be great to promote this description language.

Should be mergeable once #98 has been accepted.