Correct docs for Plugin class attributes
Fix deprecation warnings for asserts in Python 3.2+
Simplify tox config and use Python 2.7+ by default
Merge pull request #156 from jcjf/py3assert

Fix deprecation warnings for asserts in Python 3.2+.
Merge pull request #155 from jcjf/plugindoc

Correct docs for Plugin class attributes.
Merge pull request #153 from jcjf/simplifytox

Simplify tox config and use Python 2.7+ by default.