Split unittests into separate files and move them to tests/

I'm introducing a more standard way to perform unittests. Now, you simply can do this:

```bash
cd /path/to/jasper
python2 -m unittest discover
```

Furthermore, I'm splitting up unittests into multiple to improve readability.

**ToDo-list**
- [X] Split up `client/tests.py` into multiple files
- [X] Move unittests into `tests/`
- [x] Add [skip conditions](https://docs.python.org/2/library/unittest.html#skipping-tests-and-expected-failures) to tests (i. e. do not perform test if dependencies are missing. This is what we did manually with `--light`)
- [X] Update travis configuration (and integrate unittest discovery with coverage)