Move to pytest & python3

This is a continuation of the work started by @chrisjsewell in PR #23

move from unittest to pytest (some tests left in unittest form but executed via pytest)
conftest.py creates fixtures which can be utilised by all tests within the same folder or subfolder
added test_process for executing the calculation and testing it returns the correct returncode and outputs the expected files.
updated the relevant sections of the readme and documentation
plugin runs under python3
plugin is tested against aiida develop branch under python3 (and against stable + develop branch under python2)

aiidateam / aiida-plugin-cutter