Added a changelog to the documentation

I tried to fill GitHub releases gaps based on PyPI upload history.

Fixed classifier in setup.py

Version bump to 0.3.1

Bump setup file to 0.3.0

Removed pypy3 from Tox as it doesn't work with pip

Added tests for the History class

Preparing new release (0.2.7)

Added Py 3.4 and Pypy 3 as Tox targets

Changes coveralls.io badge to the SVG version

Small code review after merging pull request tcalmant#15