Adds an AVX2 option for the parser.

This gives significant performance boost on Haswell CPUs (compared to PCMPESTRI)
1.78X for bench.c up to 2X for some other on i5-4278U, with gcc 4.9.2 -O3 -mavx2 -mbmi2.
The change requires the entire parse_headers function to change -
instead looking for the next token, it now creates a bitmap of all tokens.
When compiled without -mavx2 it uses the old parse_headers.
The code was not tested much, so it might contain some bugs, I mostly
wrote it to check the performance potential of AVX2 for parsing.