Simplify the process of building a development tree.

Review Request #7629 — Created Sept. 15, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x

Reviewers

This introduces a dev-requirements.txt, which lists the pip-compatible
Python dependencies we need to use Review Board. It also installs them
when using prepare-dev.py.

Installed a new environment, and got all the development packages
specified.