Improve testing method

I think its common practice for Python projects to execute tests by calling `python setup.py test`, this was not possible before so I implemented this. Not sure if some lines in base.py are unnecessary now, so you might want to check that.

It would be great to add automatic test builds using Travis-CI to this project. If you like I can work on a solution to get a running Piwik installation onto the Travis-CI server so we can automatically test the library each time a commit happens.

Changes: 

* Changed setup.py to support the test command. You can now run tests via `python setup.py test`
* Defined test suite in `piwikapi/tests/__init__.py`