Use pytest

As for openmicroscopy/openmicroscopy, This PR migrates away from `unittest` to using `pytest` as the basic framework for testing `acc`. Modifications have been done across the entire `test` folder by:

- renaming all test files to allow discovery by `pytest`, i.e. `test_xxx.py`
- removing all occurrences `unittest` and replacing them by `pytest` structure instead (`setup_method`, `teardown_method`, `assert`…)
- using the `pytest` `capsys` fixture and parametrisation wherever possible
- extending `setup.py` to allow tests to be run via:

 ```
python setup.py test...
```
- adding the test requirements directly to `setup.py` and removing `dev_requirements`
- updating Travis script file to run the unit tests using `python setup.py test -s test/unit -v`
- updating the Readme to document the new testing commands

The [SCC-self-merge](http://hudson.openmicroscopy.org.uk/job/SCC-self-merge/) configuration will be updated to use the `python setup.py test` command as well.