[TDD] Unit tests for node/connection.py.

* Check initialization of PeerConnection and CryptoPeerConnection.
Refactor and fix PeerConnection and CryptoPeerConnection.

* CryptoPeerConnection overrides equality.
* Nickname default should be consistent with parent class.
* Make unused variable private. Enable pylint to flag future
  usage outside of class.
* Set nickname in the parent class.
* Do not initialize own logger if parent provides one already.
  The parent class properly sets the logger thanks to `self.__class__`
* Install the logger last.
* Remove unused callback argument from __init__; fix rest of code.
Remove contact tests.
Remove unused contact.py.