add python-versioneer

[warner]

This installs versioneer-0.12 to manage versioning of the python binding. The old setup.py only gets the version right if you're building from a git checkout. Versioneer also handles building from a tarball produced by "setup.py sdist" or "git archive", including the tarballs automatically generated by github's "Releases" page.

This also makes curve25519.__version__ available to runtime code.

[warner]

Oh, also, Versioneer is py2/py3-compatible. I don't think curve25519-donna is entirely py3-clean yet (I'll run some tests), but this won't make the situation worse.

[warner]

Just tested it: curve25519-donna does seem to be py3-clean.

OTOH, Versioneer is not py2.5-compatible, so this would break the outstanding pull-request that adds py2.5 compatibility. If you land this one, you should probably WONTFIX the other one, and vice versa.