Adding EC point decompression 

I'm migrating some work Zeilap and others did for **bitaddress.org** into my fork of **bitcoinjs-lib** and figured people might benefit from pushing this small enhancement upstream to **jsbn**.

Specifically, this change was implemented by Zeilap for generating EC-multiplied bitcoin keys based on an intermediate ec point.  To do accomplish this he added point decompression to jsbn.  See his fork of [bitaddress.org](https://github.com/Zeilap/bitaddress.org) for the original changes.