Polyfill for mozPrintCallback

I've created a shim for &lt;canvas&gt;.mozPrintCallback, which traps `window.print()` calls and Ctrl + P shortcuts when mozPrintCallback is not supported.

This shim does the following:
1. Intercept window.print()
2. For a window.print() call (if allowed, ie. no previous print job):
   1. Dispatch the beforeprint event.
   2. Render a printg progress dialog.
   3. For each canvas, call mozPrintCallback if existent (one at a time, async).
   4. During each mozPrintCallback callback, update the progress dialog.
   5. When all <canvas>es have been rendered, invoke the real window.print().
   6. Dispatch the afterprint event.

Effects across browsers:

- In Firefox, mozPrintCallback was already supported, so this patch does nothing.
- In Chrome, the feature works well.
- In IE11, it's impossible to prevent the default behavior for the Ctrl + P shortcut, so the dialog shows too early (when every page is ready, the correct dialog is shown though).

I didn't test the changes with Opera or Safari, but since they're both webkit browsers, I expect the same result as in Chrome.