Release version 0.0.3

Changes in this release:

* Drop `_call_requests_*` functions in module `nailgun.client`.
* Create a `requirements.txt` file, and create references to it where
  appropriate.
* Clean up the test suite. Drop `ddt` from `requirements-optional.txt`.
* Mark NailGun as being in pre-alpha, not planning. NailGun is installable and
  usable as-is, as demonstrated by Robottelo pull request 1793. It is not marked
  as anything more, though, as major design questions are still in the air.
WIP: Python 2 and 3 compatibility