Some changes to how packages are searched; the srcdir for get_package_info does not return the package info for a single specific package. Rather it it returns info for all packages under the given path (by default '.', but this can work for source trees that don't place the Python packages immediately at the root of the source (though that would generally be advisable).  It also allows excluding packages and supports that even for extension modules.
Initial import of astropy.sphinx into astropy_helpers.sphinx--mostly the same except for a few small tweaks to get it working under the name 'astropy_helpers'
This is required to ensure that when Sphinx runs it has the path to whatever astropy_helpers we're using
For some reason I got complaints that the favicon couldn't be found when I first ran the docs.  Not sure why this hasn't come up before, and it seems suspect. But this seems to fix it.
Add an option to ah_helpers to make use of git at all (i.e. for updating submodules) optional