Add a ``can_add()`` method that indicates whether a distribution matches
the collection's desired platform and Python version.  Fix a bug in
``Distribution.from_filename()`` when the filename has no Python version.

--HG--
branch : setuptools
extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/trunk/setuptools%4041038