Added support for specifying global configuration.

This code can be used to globally specify some options using the git
command `git config --global <option> <value>`. Currently supports the
git options `user.name` and `user.email`.

Currently this is just a class that hard codes the options. Eventually
it may make sense to move some of this code into a defined type. That
would make extending this code simpler. However, since I've never set
any options globally other than `user.name` and `user.email` this is
fine for now.

Also added some documentation to the base `git` class.