Github status upgrade

This PR does a number of things related to the github stats generation. The main aim is to silence the related warning in the docs build.

1. Upgrade to lates version of github_stats.py from IPython. We have always been using a copy. This has a number of advantages. Use Oauth to login to github to avoid the api rate limiter. Optional caching of requests etc.
1. Modify this to match the behaviour in the existing matplotlib script. i.e. Add header, change default project to matplotlib/matplotlib and add a note about where the script is coming from.
1. Change escaped sequences in the github output. This escapes all ` _ and *'s in the output. Some of them could be interpreted as rst markup. However, I could not find a way of doing that reliably. 
1. Regenerate the github stats with the new script.