Clean things up a little.

* Remove debug code.
* Make the Buffer argument in the encoder/decoder optional.
  If omitted, a new (empty) buffer is automatically created.
* Make the Encoder/Decoder arguments in Client optional.
  If omitted, new encoders/decoders are automatically created.
Use Symfony's DIC & add logging through PLOP.

Uses erebot/plop to handle logging & symfony2's dependency injection
component to handle dependencies (this is considered WIP).