Refactor adding bytearrays and recoding complexity

- Add bytearray support (#1 and gmr/rabbitpy#48)
- Change encode/decode type errors from ValueError to TypeError exceptions
- Remove separate codecs for Python 2 & 3
- Move codecs from pamqp.codec.encode and pamqp.codec.decode to pamqp.encode and pamqp.decode
- Deprecate pamqp.codec
- Remove weird imports from top level __init__.py, not sure what I was thinking there
- Clean up codegen a bit to make it more PYTHON3 compatible
- Update codegen/include for new codec and PYTHON2/PYTHON3 behavior
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