[Feature] Implement new voting changes

[localhuman]

Recent changes in NEO core added a new Transaction type along with a number of other large changes around voting and consensus. These changes should be implemented in neo-python.

New changes in NEO:
neo-project/neo#130

[localhuman]

I've pushed a new branch for these changes here: https://github.com/CityOfZion/neo-python/tree/feature-state-transaction

These are the non-consensus related changes we will be needing for sure:

• add neo.Core.StateDescriptor
• add neo.Core.TX.StateTransaction
• update neo.Implementations.Blockchains.LevelDB.LevelDBBlockchain.PersistBlocks to handle the new StateTransaction

There are probably additional changes needed as well, but these will provide a start.

[ixje]

reminder for #67