Minor packaging tweaks.

Renamed VERSION to __version__
Added MANIFEST.in for requirements.txt, README and LICENCE
Bumped version to 0.3.0 (we were supposed to do that as soon as we merged 0.2.0 to master)
Embedded six as rabix.common.six

In order to keep sdk-lib dependency-free rabix now carries entire copy
of six library. As a minor consequence, rabix.common can't re-export
from_json, from_url and to_json from rabix.common.loadsave because that
would cause circular dependency.
VERSION to __version__ in other cli script.
Merge pull request #34 from rabix/feature/builtin-six

Feature/builtin six