CONTRIBUTING.rst: new packaging instructions
CONTRIBUTING.rst: new packaging instructions

* Distribute both Wheel and Source
* Add a step to test distribution on test.pypi.org
* Add explicit twine instructions (and drop setup.py alternative)
  because Python recommends using it for uploads to PyPI.

Closes #225.

hemberger/MechanicalSoup,hickford/MechanicalSoup,MechanicalSoup/MechanicalSoup