Add a prepare-dev.py script for preparing a development environment.

This adds a new contrib/internal/prepare-dev.py script that sets up
the development environment for Review Board. It will automatically
do the following:

1) Create a working settings_local.py file that uses SQLite and a local
   memory cache. This uses contrib/conf/settings_local.py.tmpl as a base,
   which is now intended as the development-only settings template, since
   production users no longer need to even look at this template file.

2) Find the Djblets media directory and set up a symlink, or copy the
   media files (on Windows). This depends on a properly installed
   bleeding-edge Djblets to work.

3) Runs setup.py egg_info, which is needed for the SCMTools registration.

4) Synchronizes the database the first time.

While these are all things that can be done manually, this script makes
getting going a lot easier. It will not change existing setups, which
will continue to work fine without running this script.

Wiped out my existing configuration and ran the new script. It got me up and running to the point where I could run devserver.sh and see a working site.