Optimizations, add Tomi to AUTHORS, silence a compiler warning

Hi, I've got a few patches down the line in my private tree. Just noticed you've upgraded to github, so I'll start feeding them in from here. :)

I'm going to set up gnuais on a Raspberry Pi, which is a bit slower than the PCs I've had it on. At first gnuais took about 76% CPU, leaving little margin for anything else. By making the filter loop tighter and optimizing memory access I got it down to 46% CPU. There are some more tricks I'll add later.

Decoding results are still exactly as before.

I also added Tomi Manninen to AUTHORS, he really upgraded the GMSK demodulator to work efficiently, and silenced a little compiler warning.