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.
Merge pull request #110 from embray/reorg

Reorganize setup_helpers a bit