lmb/cli.py: added a crop feature, still need to get it to save

added draw, crop methods to img object
lmb/cli.py, lmb/core.py: fixed cropping, now saves to disk
lmb/draw.py: Fixed bounding box error, updated readme to include crop
Merge pull request #26 from lambdal/feature/crop

      lmb/cli.py: added a crop feature, still need to get it to save

Working! Beautiful!