refactored things in terms of Mask#collidesAt
completely changed the API
updated readme
wow stupid mistake not covered by the tests
new types (Mask.Box and Mask.Invert); updated tests
Mask#framedBy is obsolete now
new Mask#collidesAt, moved the old one to Mask.PBM#collidesAt
beginning to overhaul test.js, including tests for Mask.Invert
Mask.Box does not need to take x and y; you can use .at etc
some tests for Mask.Box, including one failing
fixed some bugs; Mask.Box didn't have a .clone and also Mask.collision didn't iterate correctly
hopefully this image will break some stuff
make testing height and width more robust; get 'pbm' from reading test-data
every image should be within itself
two more tests