Add HD Wallet integration (temporary and hacky!!!)

Add RPC/Websocket calls for:

getdepositscript
createseries
replaceseries
thawseries
I think there's a chance Dave may find CreateSeries too generic as a name for something that is VotingPool specific, specially given that this is exposed over the RPC API. So far all the code we wrote was in pool.go, but as the VotingPool code starts to spread into the rest of the codebase we may need to be more cautious. I'd suggest CreateVotingPoolSeries (and "createvotingpoolseries" for the RPC method name), but do check what Dave thinks as he's the one who will ultimately have to approve this. Same for the others: ReplaceSeries, ThawSeries and GetDepositScript