Support Trac "priority" field, and also support Github orgs

Here's two trivial commits to add support for:

* The "priority" Trac field.  We used that field heavily in our Trac tickets, and it was really useful to be able to bring the priorty over (as a label) to Github issues.
* Allow the target github repo to be in an organization.  If "org=foo" is supplied in the config file, look for the repo in that org (vs. the user's personal repo named "foo").

These two features were extremely helpful in porting the Open MPI project (http://www.open-mpi.org/, https://github.com/open-mpi/) Trac instances to Github.  We hope to be fully migrated over to Github within the next few weeks.

Many thanks for tratihubis!