Migrate away from BigInteger.js to either BigInt or JSBI polyfill

[plcp]

We have #16 scheduled to be merged that introduce consensus signature verification, and thus a need to do some nice RSA ourselves. This requires to handle big integers in Javascript, a feature exposed through the BigInt API... only available to few browsers and not even supported (yet) by Firefox. We thus need a third-party provided polyfill.

The current one in use by #16 is BigInteger.js, seems to be well maintained and even though by its author for some cryptographic applications.

However, I think it'll be sensible at some point to migrate away from this library to either use a native BigInt-based implementation… but as it may take some time to be widely supported, I'll recommend the GoogleChromeLabs polyfill (or jsbi) that is purposely written to ease the migration towards BigInt.

Note that this also can provide a more stable third-party to rely upon until we can avoid using one.