v0.4.0

Node

• Broadcast blocks to network if the local blockchain has higher index,
• Remove the node from the network when it is closed
• Fix a bug which caused some validations to be skipped if the chain is being reseted
• Improve miner so that as soon as a new block is added by network, It moves on
  to the next index.

Code

• Add Block.calculate_hash, {Network, Node}#last_block
• Rename {Network, Node}#download_block to {Network, Node}#block_at