Calculate POCSAG syndrome with a bitslice algorithm

Sort of like the bitslice triple-DES you use in a password cracker
(see John the Ripper's code), we turn the problem sideways and do
32 attempts at once, in parallel. This turns out to save quite a
bit of CPU time when we're correcting POCSAG errors.
Merge pull request #6 from arekinath/bitslice_pocsag

Calculate POCSAG syndrome with a bitslice algorithm