Optimizations for best_fit

- Precompute the max bit sizes for each QR code version (size) and error
  correction level.
- Replace the slow bisection with a guess-and-check that finds the best
  fit in a much smaller number of tries.
util.py: remove BestFit class

Clever as it was, no longer needed since we're now searching a
precomputed table of integers.
Merge pull request #56 from welshjf/master

Best-fit optimization