Support xpubkey lookup + BIP32

[JornC]

A much-requested feature in various bitcoin communities (though not specifically requested of this block explorer) is the ability to lookup the 'balance' of the addresses that may be derived from an xpubkey.

Now that address indexing is being supported by this block explorer, the ability to do this has become closer to reality.

All we need is BIP32 support and some infrastructure design.