want bzrlib.initialize() to do all typical setup

As a followon from https://code.launchpad.net/~mbp/bzr/499637-default-uifactory/+merge/17358 and bug 499637:

For a non-bzr bzrlib user, there are a few things that need to be done after importing bzr to get typical default behaviour: load_plugins, trace.enable_default_logging, install_bzr_command_hooks, make_ui_factory()

This is a bit problematic to do at import time because of Python circular import limits, and because not all of them will always be wanted.

It should probably take options to let some of these be skipped.