feat(tox): add support for testing with tox

This adds notes in the README and a tox.ini file for use with the tox
test manager.

This helps greatly towards migrating python-memcached towards python
3.3+ compatibility.
Merge branch 'tox_support' of https://github.com/cabrera/python-memcached into cabrera-tox_support

Conflicts:
	README.md
	tests/test_setmulti.py
Merge branch 'cabrera-tox_support'