y4m input support

Can now read (not write) y4m files and could accept y4m streams on stdin.
fopen in binary mode
add license conditions
deduplicate some code in motion search by pushing it into its own function and inlining it
restructure brute-furce motion search to be a lot more compact and more readable (IMO). Also gives a small speed boost.
some comment fixes
save a minor amount of arithmetic operations during motion estimation
implement the Tree Step Search (TSS) algorithm for motion search, proposed by Koga et al. 1981. This dramatically speeds up motion search, and encoding in general. On an AMD E-450 nettop, encoding the sign_irene sequence now takes about 16.6 seconds, instead of 27.3 seconds with the exhaustive search.
correct vector bounds for TSS motion estimation. Don't test the center position again and again.
rename BruteMotionEstimation to MotionEstimation as brute-force search is not the only or desirable motion search technique.
test motion vectors in a presumably more cache-friendly way (outer loop y, inner loop x). Seems to be tiny bit faster.
more appropriate initial step size for motion estimation
Merge remote-tracking branch 'vproject/y4minput'
speed up quantization by letting the compiler generate fast integer divisions for the most common divisors. Also reformat the source file.