Library for extended public keys.

[vulet]

We could offer a slight level of privacy to domain/webpage owners by allowing them to use extended public keys to generate tip addresses. This could be done by creating a small library that serves random addresses from an xpub. The process would look like

1. server owner puts xpub on back-end
2. server owner puts the amount of randomness they require
3. front-end is served a "random" address

Alternatively, we could have the library generate a new address every time a wallet gains balance.