Moves the Protocol Specification to markdown

This pull request converts the [Rosbridge 2.0 Protocol Specification](http://kforge.ros.org/rosbridge/trac/raw-attachment/wiki/WikiStart/rosbridge%20protocol%20specification.pdf) to markdown and adds to the repository.

The original 2.0 specification was a PDF on kforge. The PDF had some outdated links and details and I don't fully trust kforge as a document repository. Also, as a markdown file in GitHub, we can link to specific sections when referencing for other people.

Note: I spent most of the time converting the PDF to markdown and updating a few links. Other parts of the document may still need updated.

I discussed with @odestcj. He mentioned possibly moving the document to another repo. I'm sending it here for now until we discuss other locations.