Added username and password as command-line options.

The primary change here is that you can now specify username and password as command-line arguments. I made some changes in the Gerrit class to simplify its initializer, at the expense of introducing a factory function. My experience is that this is a better design in most cases, but I'm happy to remove it if you think it's a step too far.

Another change I considered was to move the environment-variable handling code out of `gerrit.py` entirely. Having if there feels like a conflation of core functionality (i.e. gerrit interaction) with user-interaction stuff (i.e. checking environment variables.) But I worry that a change like that might make things more obscure than necessary.