Cross-domain Canvas limitations: add try/catch and document it

[gregallensworth]

Note that Canvas does have cross-domain issues much as does AJAX. Loading tiles from a remote origin will not work properly unless CORS headers are set.

• Add try/catch to PixelFilter.js at that getImageData() around line 99, so it gives a more useful error message
• Document this in the README so it's pretty clear, cuz folks likely don't know offhand

[gregallensworth]

This round of commits greatly improves the cross-origin situation:

• Documented it and gave links to other places to learn about CORS
• Extended the _createTile() function to add the .crossDomain="Anonymous" attribute, so the tile is "CORS ready" if headers are available.
• Added exception handling around getImageData() so that if it does fail, a more useful error message is given.