Change ``clean`` rule to remove generated docs

Made necessary by https://github.com/astropy/astropy/commit/b94fd38e5613c838c41ef5fa57ed9ba8289b5046
Update to newer ez_setup.py
Add the current version of ah_bootstrap--for the tests themselves we will always copy over the latest version of ah_bootstrap (so that we can test ah_bootstrap itself), but there should still be a default copy of it in the package-template repo as well
Configure the package-template to use astropy_helpers.  By default includes astropy_helpers as a submodule. Yes, astropy_helpers is now a submodule of package_template which is a submodule of astropy_helpers
update the astropy_helpers version to include fix from c3f841b9e099f506a751b8a6971b15e7e3d6590a
Update the astropy_helpers version used in the package_template
Update the astropy_helpers version again; use the auto_use feature of ah_bootstrap.
Switch to using astropy_helpers for docs build.
Updated astropy_helpers version (included fix for Python 3)
Merge pull request #42 from mwcraig/patch-2

Change ``clean`` rule to remove generated docs
Update the git submodule path to use http instead--hopefully this will work then on Travis
Updating to the actual astropy-helpers
Merge pull request #46 from embray/astropy_helpers

Astropy helpers
Updated astropy-helpers to latest version
Merge pull request #61 from astrofrog/update-astropy-helpers

Updated astropy-helpers to latest version
Don't override package data after it's been determined from
setup_package.py files