Implement BIP43 and BIP44

[sbuss]

Note that this requires interacting with the blockchain in order to scan an address for transactions, as detailed in the account discovery section of BIP43. Therefore support for this requires a lot of work (though it's all work I've been meaning to do for a year now...)

Partial support can be added for generating addresses given knowledge about the last used account number, but that's somewhat dangerous and could result in addresses with potentially unrecoverable transactions because correct implementations stop scanning for valid account numbers when it finds one without any transactions. (Though I may not understand this correctly, since it seems that you could create an address for somebody to pay into, but they never do, and now you have to either reuse that address for a new user, pay a small amount into that address yourself so address discovery will continue to work, or create a new wallet)

https://github.com/OpenBitcoinPrivacyProject/bips/blob/master/bip-0043.mediawiki
https://github.com/OpenBitcoinPrivacyProject/bips/blob/master/bip-0044.mediawiki