Change data representation of BigInteger

Store the integer parts of a BigInteger in an array instead of directly
in the BigInteger object. This improves performance of RSA key
generation on most browsers (Firefox, IE 10, Safari) by an order of
magnitude. Performance in Chrome stays more or less unchanged.
Merge pull request #19 from AchimO/master

Massive performance boost for RSA key generation in Firefox, IE 10, Safari.

Thanks @AchimO!