Add a get_change method to SCMTool and HostingService

This change adds the final piece of integration API on the backend that we need
for post-commit reviews--the ability to fetch the commit message and diff from
the version control system given a revision ID. This is implemented for SVN and
GitHub, just like the get_branches and get_commits methods.

Testing done:
- Ran unit tests.
- Used this with my post-commit UI for both github and svn repos.

Reviewed at http://reviews.reviewboard.org/r/4284/