Listener refactor

### Motivation:
It turns out everyone uses `TransportLayer` as a dump for every method or attribute whose proper place is undetermined. As a result, this class is bloated with all sorts of code: managing sockets, listening for requests and handling various pieces of crypto code.

This is the first of a series of attempts to reduce the bloat of `TransportLayer` and factor the code into several smaller and simpler classes, more amenable to testing and maintenance.

The new `Listener` classes mirror the pattern of `PeerSender` and `CryptoPeerSender` but are designed cleanly and follow good practices such as defining all members in constructors. They are also completely independent from `dht.py` and `transport.py`, whereas `Sender` classes still have this problem and should be refactored in the future.

### Changelog:
* Factor classes `PeerListener` and `CryptoPeerListener` out of `TransportLayer` and `CryptoTransportLayer`, respectively.
* As the `Sender` classes get a `dict` that is then encrypted, compressed and sent, the listener classes do the inverse actions and receive, decompress and decrypt the messages, relaying the sent `dict`s to the receiver.
You are already inside a `try` so go ahead and access `r.text` without a check; catch `AttributeError` in the `except` clause and handle accordingly. See: [EAFP](https://docs.python.org/2/glossary.html#term-eafp).

By default,  [strip](https://docs.python.org/2/library/string.html#string.strip) strips whitespace. Refactor as:
```
return r.text.strip()
```