This converts the testing infrastructure to tox for both locally
testing and travis. This is nearly equivalent to the previous testing
with the only exception that linting fails with the first tool to fail
and won't process all of them.

Slightly tricky thing is that tox resets *all* of the environment for
it's subprocess runs by default. A couple of the dependencies we have
will not install in non UTF8 locales: temper-python & XBee.

Changed CONTRIBUTING.md to refer to requirements_all.txt instead of requirements.txt

1) Added visibility documentation to the CONTRIBUTING.md documentation. 2) Pylint fixes to homeassistant/helpers/entity.py

Updated icon list url