Overriding __redirect_uri method in GitHubAccount

This prevents 'redirect_uri_mismatch' errors during OAuth authentication, by using the redirect_uri specified in curator/private/config. Otherwise the opentree server might by proxied or otherwise set up to report itself as 'localhost:8000' and build a wildly mismatched URL.
Merge pull request #81 from OpenTreeOfLife/simple-curation-app

Overriding __redirect_uri method in GitHubAccount