Sort of updated this test to work in astropy_helpers.  Not sure why I wasn't seeing this failure before.
Add an option to ah_helpers to make use of git at all (i.e. for updating submodules) optional
Something that occurred to me--options for use_astropy_helpers can be
read from setup.cfg.  As such we can enable an option to not have to
manually call ah_boostrap.use_astropy_helpers in setup.cfg.  Instead
just importing ah_boostrap alone will run its '_main()' function, and if
setup.cfg contains an '[ah_bootstrap]/autouse = True' then it will automatically call use_astropy_helpers() (along with any options provided via setup.cfg)
There was an ugly bug where if using astropy_helpers is bootstrapped from a .tar.gz file, the boostrapping process itself involves running astropy_helpers' setup.py, which in turn runs astropy_helpers.setup_helpers.register_commands, which through add_command_option can modify classes in Distutils.  We go ahead an leave the modifications in place (for now) but remember that they were made by astropy_helpers so that an attempt isn't made to add them a subsequent them (leading to a RuntimeError further down).  This is an ugly hack but a good enough workaround for now.
First steps toward an "upgrade" functionality--ensure that a
Distribution object for the astropy-helpers that will actually be used
at runtime is available on pkg_resources.working_set.  We will use this
later to determine if a newer astropy-helpers exists than the one that
will otherwise be used.

Also fixed an issue where installing from a .tar.gz caused a second,
superfluous attempt to download the package (due to a missing return
statement)
Fixed a test that was failing, though this is a little hacky and unreliable--not sure how long this will stand like this.
Use .as_cwd() where appropriate
Some big restructuring toward supporting automatic upgrades.  This now focuses on getting a pkg_resources.Distribution object representing the version of astropy_helpers we actually want to use.  Once that is found (if it is found) it uses the .activate() method to actually add itself to sys.path so that astropy_helpers can be imported.  Along with this comes actual preliminary support for auto-upgrades, and some new tests.  It also adds an auto_upgrade option to use_astropy_helpers that allows auto-upgrade to be disabled if necessary (most of the functional tests set it to disabled by default).
Some significant speedups by avoiding running egg_info every time ah_bootstrap is used (though it may be needed the first time around--will suggest bundling astropy-helpers with its egg-info/dist-info already written