mutt_ldap.py: Rename with underscore for package import

If I'm breaking Mutt-LDAP as a stand-alone package, I should probably
make it importable.  Even if nobody else imports it, we can still want
this for setup.py-based installation.
setup.py: Package Mutt-LDAP using distutils
.gitignore: Ignore *.pyc (generated when you run setup.py)
README: Every stand-alone project desrves one

This is based on my usual template.  I also simplified the docstring
in mutt_ldap.py to stay DRY, since that information is now in the
README.