Misc. interface improvements.

- Renamed `CircuitManager` to `TransportManager`, and put it in `oppy.transport.manager`.
- Began implementation of the `TransportManager` class.
- Made initial implementation of the `MultiForwarder` class.
- `OnionSocksProtocol` now inherits from `OnionStream`.
- Made `OnionConnection` stub.
- Made `OnionCircuit` stub.
- Made `CellReceiver` and `CellSender` inherit from `Forwarder`.
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Merge branch 'interfaces' of https://github.com/nskinkel/309Project into interfaces
Removed a config variable. (server descriptors are not cached.)