Electrum style key derivation

I wrote a class for doing key derivation from an Electrum's master public key.

I wasn't sure where to put this so I created an Electrum file and class. It's a WIP. Right now it can only do derivation from a master public key. 

You can test it by creating an Electrum wallet, copy/pasting your Master Public Key (Wallet/Master Public Key) in `examples/ElectrumMPK.js` and checking that the pubkeyhash that is printed matched your first receive address in the Receive tab.