Unit test node/transport.py and expand tests of node/contact.py.

### Changelog:
* Move tests of `node/transport.py` to `test` folder. Refactor them as unit tests.
* Test `Contact.__repr__` is implemented and `Contact.__str__` derived.
* Drop `run_test.sh`; execute unit tests via `nosetests`. Ref: #82.
* Minor style improvements.
Use `mock`