Initial implementation of astropy_helpers

This starts to add the important pieces of astropy_helpers.  It ports `astropy.version_helpers` and `astropy.setup_helpers` into the new package and modifies them to work in the new context, without access to the `astropy` package itself.

It also adds some initial infrastructure such as the `ah_bootstrap.py` script that other packages will use to bootstrap astropy_helpers when using it.

This should serve as an initial reference implementation for use with the first draft of the astropy_helpers APE.  Rather than clone my fork of this directly, check out the pull request in astropy/astropy#1563.  The branch I based that pull request off of pulls in this version of astropy_helpers (from my fork) as a submodule.