Gridcoin support

[grctest]

Hey,

I've been interested in Blockstack for a few months now, I would really like to investigate the creation of a Gridcoin driver for blockstack-virtualchain.

I'm hoping to improve Gridcoin's voting mechanism with Blockstack; we use the voting mechanism for foundation expenses, the BOINC project whitelist and for community polls (voting on community decisions).

Blockstack would provide our entire community the ability to extract the voting data and potentially to hold persistent polls for the whitelist.

Cheers guys

Relevant issues:
hirosystems/stacks.js#92
stacks-network/pybitcoin#54

Links:
https://www.gridcoin.us
https://cryptocointalk.com/topic/46957-porting-blockstack-from-bitcoin-to-gridcoin/ (Initially investigating porting blockstack to gridcoin a couple months ago)