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 patched in through the use of `src/bigi.js`.
This provides the utility functionality for `Buffer` -> byte array conversions currently required for several parts of the library.
This is undoubtedly not the ideal solution for this, however provided we can merge a fix for  upstream to support Buffers, this is no less 'hacky' than the previous amendment that existed.

Although this isn't actually a replacement (#24), as `bigi` is actually just JSBN anyway; it does externalize the testing and maintenance for this module, and therefore place less responsibility on this library to maintain the dependency independently.

This pull request should (probably?) not be merged until the [**the tests that previously existed are merged up stream.**](https://github.com/cryptocoinjs/bigi/blob/master/test/bigi.test.js)

The test coverage of JSBN has always been poor, and this pull request does not change that (however it should make it easier in the long run).

**edit:** as in #24 , this may be a good opportunity for a better big number library.