Github client module

A python module for wrapping Github API calls, for easier development and testing outside of web2py. This can be imported in python or run directly from the command-line (but should be passed an OAuth token for best results):
```bash
cd api.opentreeoflife.org/modules
./github_client.py AUTH_TOKEN
```
The GET method in the default controller now calls this module instead of the helper method ```_get_nexson```. The latest version of OpenTreeOfLife/opentree@76a149f12836a3262f0c1a93ccc15958e95486b4 will use this to fetch studies into the curation tool (study edit page).