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.
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