sjcl.random improvements

This change covers what we talked about in issue #77.   There isn't a lot of code here so it should be pretty easy to review.  The init() method is new.  The entropy pool is always initialized to 48,  and if you think it should be 0 then I will be the last person to criticize you for being paranoid.   I am happy to discuss any of the changes I made and there is still room for improvement.

Here is a list of changes:
Better support for mobile devices.
Added new sources of entropy.
Supports saving and restoring entropy from localStorage.
Stronger randomness at initialization (especially for non-chrome browsers!).