setup: better version handling

* Replicates version handling from Flask-Registry. (closes #4)

Signed-off-by: Matthew Dillon <mrdillon@alaska.edu>
Reviewed-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
tests: tox support

* Adds support for tox testing. (closes #5)

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>