Addition of HeaderDiff codec
Merge pull request #17 from hruellan/HeaderDiff

Addition of HeaderDiff codec
Removed references to the function which strips values out of headers--
that is already done before it is fed into the compressor.
Fixed some bugs I just recently introduced w.r.t. instantiating stuff,
      this also made the code more simple. Gotta love it when that
      happens.
Fixed a bug w.r.t. not having implemented the __nonzero__ method on the
LRU class.