Add default host as configuration option (hub.defaultHost)

[rjbell4]

This adds a git configuration option to set the default GitHub host. We are deploying GitHub Enterprise, and our developers will use our GitHub Enterprise server the vast majority of the time (99+%, etc.), with access to github.com probably limited to a very few select situation. As such, we'd prefer a more "sticky", uniform way to nearly always use our GitHub Enterprise server.

I've tried to model this off of what I see other code doing, etc. I have had trouble running some of the tests, though, even without this case. Therefore, it's highly likely that these tests need some work. I'd really appreciate it if someone could work with me on that. We really need this simple functionality, and would really like to have it in the official hub source.