Add DPOS consensus

Find a way to incorporate DPOS consensus into the ABCI server. One of the ways to achieve this can be something like this:

- Add a "voting" transaction and let people cast their votes in favor of specific validators
- The global state can store a sorted list of top 21 validators
- The `EndBlock` callback can be used to update the list of validators so that this list changes each time a vote is cast.

Resources:

- DPOS: https://bitshares.org/technology/delegated-proof-of-stake-consensus/
- How Tendermint works: http://tendermint.readthedocs.io/en/master/introduction.html