Add vimeo rst directive with doco

I created a vimeo rst directive based on the existing youtube directive, but added the ability to automatically find the video height and width from the vimeo API. When I added the doco I added an example with parameters, which should also give people hints when using the youtube directive.

I wasn't sure which version of python you are targetting, so the API call will only work with python 2.6 (it requires the json module) but it will silently fall back to default size if json is unavailable. I tried to keep the coding style as close as I could to what I've seen in the project.

Comments welcome