Fixed position nav

This changes the sidebar nav in the default bootstrap template to be fixed to the side of the page. This makes it possible to scroll through the nav and the content area independently, which is useful for larger API's. I have not regenerated the examples in this pull.

I obviously don't know what your priorities are for the project, but feel free to merge this branch if it seems appropriate. These are changes that I have made while creating my own API docs.

Thanks again for the awesome tool!