Removal of JSBN

In this pull request JSBN has been replaced with the module [bigi](https://github.com/cryptocoinjs/bigi).

An extra method `BigInteger.fromBuffer` has been has been patched in through the use of `src/bigi.js` as the intermediate `Buffer` -> byte array step currently required for much of the library.
This is undoubtedly not the prettiest solution, but hopefully we can merge the intended fix for JSBN into bigi upstream rather than maintain the entire dependency separately in this repository. 

Although this isn't actually a replacement (#24), since `bigi` is actually just JSBN anyway; it does externalize the testing and therefore place less responsibility on this library.  A bonus for all.