Don't warn on empty non PEP 440 versions

Empty versions which are not PEP 440 are more likely to be the
fault of setuptools internals rather than anything a user has done
so these warnings are needless spam in that situation.
Upgrade packaging to 14.5

* Normalizes release candidates to 1.0rc1 instead of 1.0c1
Merge pull request #20 from dstufft/dont-warn-empty

Don't warn on empty non PEP 440 versions
Add a PEP440Warning to make it easier to silence these warnings
Merge pull request #21 from dstufft/subclass-warning

Add a PEP440Warning to make it easier to silence these warnings
Merge pull request #22 from dstufft/upgrade-packaging

Upgrade packaging to 14.5
Update changelog
Bumped to 8.1 in preparation for next release.
Added tag 8.1 for changeset 3f87370b6863
Bumped to 8.2 in preparation for next release.
Merge with 8.1