Replace bigi with digit-array

[```bigi```](https://github.com/cryptocoinjs/bigi) has a lot of code that is not needed for base conversion.
~~[```digit-array```](https://github.com/deckar01/digit-array) is a much lighter library that specializes in converting between arbitrary bases.~~

This change should also make encoding and decoding faster, because [```bigi```](https://github.com/cryptocoinjs/bigi) uses a large intermediate base internally.
~~[```digit-array```](https://github.com/deckar01/digit-array) performs direct base conversion, which should cut the conversion computation in half.~~

This PR replaces ```bigi``` with a small amount of pure JavaScript.

Fixes #2.