Added -s parameter to stop enum parsing at a given enumerator

--HG--
branch : pure-cffi
fix Packet and merge it into FFPacket

It's a start. However:
- the constructor is still messy.
- maybe we need separate constructors a-la dict.fromkeys.
- intializing from python-provided data (opposed to libav* provided
  data) is still a bit clumsy and likely involves too much memcpy().
- the data-related unit tests are broken (WIP)
- pylint evalution is likely dropped, as coverage did (temporary
  but still annoying)

--HG--
branch : pure-cffi
fix payload handling in Packets

Packet must take care of and track the effectively used data
when it is user-supplied (through python code not from libav*)
and act accordingly for hashing, equaliy check and bytes conversion.

--HG--
branch : pure-cffi
first repr()s

--HG--
branch : pure-cffi
Merge ../pyrana