Add a --callback option

Addon providers may need to access the data from the callback as part
of the initial setup, so it's useful if kensa can stub that out too.
Add support for other heroku-compatible upstreams

They both already recommend using kensa for the basics,
this commit lets you use kensa for testing in addition to
manifest management.
Merge remote-tracking branches 'github/callback' and 'github/upstreams'