Support for configurable base: {10, 16, 32, 36, 62, 64).

I needed the ability to support different bases (character sets) for my short IDs.

Included in this pull request are bases 10, 16, 32, 36, 62, and 64.

The only additional dependency is [bignum](https://github.com/justmoon/node-bignum) which, in turn, doesn't require any additional dependencies beyond Node.

jpap