Update readme, add long_description to setup.py

Reformat the readme from markdown to restructured text. This should allow it to
play more nicely with PyPi. Make the readme link to ReadTheDocs. Use the readme
as a `long_description` in `setup.py`.
Release version 0.0.2

Version 0.0.2 contains a single useful module: `nailgun.client`. This client is
a port of `robottelo.api.client`, with only a minimal set of changes on top of
what robottelo provides.