Fixes an issue with the build_sphinx test where it wasn't restoring the cwd correctly, and was also copying more of astropy_helpers than was necessary.
Fix a mostly unrelated issue with build_sphinx that seems to be affecting the Travis builds.  Don't be as strict about where the 'build succeeded' message appears in the Sphinx output.  (Why do we even do this? Does sphinx not produce a usable return code?)
Merge pull request #111 from embray/fix-build-sphinx-test

Fixes an issue with the build_sphinx test
Broke apart and reorganized much of astropy_helpers.setup_helpers.
Most of the code related to implementing individual distutils commands
has been moved into an astropy_helpers.commands/ subpackage, with a
module devoted to each command.  Some of these modules are very short
and seem a bit silly, but this organization is simply to match the
organization of distutils and setuptools.  It also makes it easy to find
where a given command is implemented.

Also moved most of the functions related to introspecting distutils into
a new distutils_helpers module.