Version class replacement for versioneer

Simple param Version class usable by any param-based project instead of versioneer. Supports (approximate) version comparison between commits during development (based on the number of commits since the last version tag).

Tested with Python 3 and tested integration with Topographica using 'flakes', 'unit' and 'snapshots' tests. Not tested on Windows (yet).

I can write some unit-tests for this class if we introduce a 'tests' directory to param. Other than that, we simply need to make sure all the git version tags begin with a 'v' and it should be ready to go.

Jean-Luc
https://github.com/ioam/param/issues/62