Log Message:
Update net/py-hpack to 2.1.0.

Changes:
2.1.0 (2016-02-02)
------------------
**API Changes (Backward Compatible)**
- Added new ``InvalidTableIndex`` exception, a subclass of
  ``HPACKDecodingError``.
- Instead of throwing ``IndexError`` when encountering invalid encoded integers
  HPACK now throws ``HPACKDecodingError``.
- Instead of throwing ``UnicodeDecodeError`` when encountering headers that are
  not UTF-8 encoded, HPACK now throws ``HPACKDecodingError``.
- Instead of throwing ``IndexError`` when encountering invalid table offsets,
  HPACK now throws ``InvalidTableIndex``.
- Added ``raw`` flag to ``decode``, allowing ``decode`` to return bytes instead
  of attempting to decode the headers as UTF-8.

**Bugfixes**
- ``memoryview`` objects are now used when decoding HPACK, improving the
  performance by avoiding unnecessary data copies.