Farewell, BinaryWriter

Version v0.14 had started working on the new .to_bytes() method to dump the BinaryWriter and its usage on the .on_send() when serializing TLObjects, and this release finally removes it. The speed up when serializing things to bytes should now be over twice as fast wherever it's needed.

Other internal changes include using proper classes (including the generated code) for generating authorization keys and to write out TLMessage's.

For bug fixes, this version is again compatible with Python 3.x versions below 3.5 (there was a method call that was Python 3.5 and above).