Documentation for namespace packages, working sets, and supporting custom

PEP 302 importers.  Once the "Distribution" class is documented, this will
be a complete API reference for pkg_resources.

--HG--
branch : setuptools
extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/trunk/setuptools%4041196