Add ability to broadcast and receive blocks

[jcopley]

Create an endpoint to receive a complete block.

When a block is received:

1. The block index(?), transactions and previous hash are checked to verify that it is a valid addition to the current chain, and not a duplicate. If valid:
2. The block is added to the current chain.
3. Current transactions are purged of any transactions that appear in the new block.
4. The block is re-sent to remaining nodes.

When a new block is mined, it will be sent to other nodes.

[lancekrogers]

Add ability to replace tip of the chain instead of replacing the whole chain