Finds convex hull when wearing a new image.
Add .clip() method for sprites.

Also makes sprites not-perfectly-transparent by default.

Also refactors and fixes some small issues with the new hull-computing
code (thanks for the code alex), and adds a test.
Deal with cross-origin restrictions on bit access, and remove openicon urls.
Slight optimization.
Merge pull request #88 from PencilCode/clip

Add clip method and automatic convex hull hit-testing for transparent images.