Fix IE not implementing addEventListener/removeEventListener

This should fix issue travist/jsencrypt#10. The fix is simple if window.addEventListener exists it will be used otherwise if attachEvent exists it will be used instead. If none of them exists (browsers that should die) simply ignore it and move on, the entropy will be generated later by Math.random.

Sadly i cannot test this, so if someone with IE (most notably IE 7/8) can test this it would be great

Antonio