Auxiliary Proof of Work logic

Add the logic and configuration toggles to allow the bitcoinjs-server core to be used for altcoin chains that allow Auxiliary Proof of Work (AuxPOW) (a.k.a. [Merged Mining](https://en.bitcoin.it/wiki/Merged_mining_specification)) to mine blocks on multiple chains at once.

This patch has been tested on the Namecoin altcoin ([example](https://github.com/MidnightLightning/namecoinjs)), and successfully syncs past the 19,200 block height where AuxPOW mining started.