This introduces an abstraction layer for installing packages using a

variety of methods. This includes:

apt-get build-dep
apt-get install
brew install
curl <url> | pythonX.Y
pacman
pip install ReviewBoard[...]
pip install
yum install
zypper install
Arbitrary shell commands

This is all done by running run_install_method() and passing in the

appropriate arguments. It will be used for package installation in an

upcoming change.

Unit tests pass.
Tested each of these with the rest of the upcoming installer changes.