Resolve #41: Implement latest spec changes

https://github.com/alobbs/libhpack/issues/41

Since HPACK specification draft 6 there's been some changes and they have all been addressed in this commit.

List of changes:
- Updated format to include literal headers that must never be compressed.
- Updated Huffman table.
- Updated Huffman test cases.
- Updated static header table (adding and removing status values).
- Updated header examples.