Abstract GUIDs. Properly support unicode GUIDs.

### Motivation:
PR #686 implements `__eq__` for `node/connection.py::CryptoPeerConnection`. The implementation was borrowed from that of `node/contact.py::Contact`, which was deleted in the said PR. However, the implementation was not correct: it mishandled the case where `unicode` GUIDs are used. This PR fixes that. It also abstracts the GUID out of `CryptoPeerConnection` into a separate file.
  
### Changelog:
* Factor `AbstractGUID` out of `node/connection.py::CryptoPeerConnection` and `test/test_kbucket::AbstractGUID`.
* Add `unicode` support. 
* Supply a complete testsuite for the new class.

### TODO:
* Fix documentation.
I'm not sure I feel very comfortable with this contract. Can we talk again about why we need `__eq__` between strings and objects? It feels wrongly designed... Perhaps we should rethink this.