Working integration of GitHub OAuth2 for login

This uses the GitHub user API to fetch and 'shadow' core GitHub user properties in web2py's auth_user table. This stores a working GitHub auth token for API operations, for proper attribution. Adding user entries in auth_user allows us to use web2py's normal mechanisms for setting groups, roles, and permissions.
Merge pull request #80 from OpenTreeOfLife/simple-curation-app

Working integration of GitHub OAuth2 for login