Add viewcode extension to allow for links to source in API docs

This creates the [source] links in the API documentation sections.  You can read more about this Sphinx extension [here](http://sphinx-doc.org/ext/viewcode.html).  There could be a reason this is NOT already added so let me know.  I find it useful to have this because often times I want to easily jump to the source in the HTML docs.  Of course, I typically just use this to quickly find the file and open it in a real editor.

It might be worth noting this functionality was added in Sphinx 1.0, which was released in July 2010.  I'm not sure what the rules are on backwards compatibility for the documentation build.

I tested this with Sphinx 1.1.3 and it worked without any issues.