Move documentation generation into WeaselDiesel

It's currently in https://github.com/mattetti/wd-sinatra, but it makes more sense in WD itself. This isn't ready to be merged, we should probably discuss whether or not this is the right approach.

In wd_sinatra, it was defined as a rake task. Here I changed it into a weasel_diesel CLI via Thor. It takes 1 required argument, which is the source path, and a second optional argument, which is doc destination path. I had to stub in an implementation method in WeaselDiesel, so that the doc generator could parse the web service definitions.

I ran it against my local test wd_sinatra app, and it seems to work.