Add the field mirror_path to RepositoryResource.

This adds the mirror_path field to RepositoryResource. This field
was previously not exposed in the API.

Reviewed at https://reviews.reviewboard.org/r/4897/
Update print statements to use the new function syntax.

In python 3, print is a function rather than a statement. This is helpfully
included in python 2.6+ with a __future__ import.

Several of our print statements existed inside management commands. The proper
way to output inside of a command is with self.stdout.write (or
self.stderr.write), so I've updated all of those.

Testing done: Ran unit tests
Reviewed at https://reviews.reviewboard.org/r/4904/