Voting Pools - Deposit side

This adds a new votingpool package containing the basic data structures to implement voting pools and DepositScriptAddress(), which generates a multi-sig script for the given series/branch/index and returns its P2SH address.

As there's no generic API to access the wallet's DB (yet), some of our code has to live in the waddrmgr package (in the votingpool_bridge.go file) so that it has access to the DB API.

(I'm the one submitting this, but this is really a joint effort from myself, @larshesel, @teknico and @jimmysong, so any praise/blame should be shared between all of us)
Yep, we already did that in the branch where we're implementing the withdrawal process (https://github.com/monetas/btcwallet/blob/vp-withdrawal/votingpool/error.go). Would you be ok with us keeping these errors untouched for now as our next PR will move them over?