Use window.crypto.getRandomValues() as a seed if available

Comments welcomed, particularly about how many bytes I should be reading (I'm assuming I should fill the first pool?), and if it should even bother with any of the other entropy sources if this one exists (this is supposed to be "strong").

Tested with Chrome (supported), Safari (supported), Firefox 20 (not supported) and Firefox 21 (supported).

Fixes #10