Redirect Tree view to Index.ipynb if it exists

First pass on #181, adding an index page.

I've deployed this to qa and have two samples:

## GitHub repository

http://qa2.nb.minrk.net/github/rgbkrk/index.ipynb/tree/master/

## Gist with index.ipynb

http://qa2.nb.minrk.net/gist/rgbkrk/8653938

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While testing this out, it seems like we don't have redirection to a tree enabled (have to know the magic behind nbviewer's URLs to list a repo). Am I missing something here?