Support PEP 440 local version identifiers

[jeremy avnet]

Parsing a string with a local version identifier results in an AttributeError. As an example, try "1.0+abc.5" which is in the PEP 440 doc.

https://www.python.org/dev/peps/pep-0440/#local-version-identifiers

[Roy Wright]

Thank you!
Just pushed 0.2.0 to github. Added PEP440 local version identifier support. Please examine the unit tests to see if I'm covering locals well enough.

[jeremy avnet]

Thanks for adding support, @royw.