Added isBigInteger static method

Uses duck-typing with a special property to detect if its a `bigi` instance and optionally match the version number.

This is needed because `instanceof` checks doesn't work when multiple `bigi` libraries are used, which is quite common with npm.

Specifically, I had an issue due to my application specifying `bigi` as its own dependency and creating `BigInteger`s with that, while the `ecdsa` library relies on an `instanceof` check with its own `bigi` dependency to verify that a provided argument is an `BigInteger`. Following up with a patch on `ecdsa` to use the new method.