Add support for canvas.toBlob

Several order of magnitudes faster than canvas.toDataUrl, about 10x fast in my measurement in Firefox. The most important part though is that it doesn't freeze the UI while running (several seconds with 10MB images).
Currently this method is only supported in Firefox and IE10+, but at least Google seems to be working on adding support. There is a polyfill here https://github.com/blueimp/JavaScript-Canvas-to-Blob but it actually uses canvas.toDataUrl in the background so there's no real reason to add it in.