Refactor node module

v0.3.2

Merge remote-tracking branch 'origin/master'

Some fixes about transactions

1. Check duplicate transactions when add incoming block.

2. Do not broadcast bonus transaction when a new block is mined.

Refactor to `event-driven`

Rewrite the way nodes run using a 'event-driven'(I'm not sure) pattern. No Mutex and no logic in main now.

Add broadcast

Nodes update automatically when they receive messages from other nodes.

Really naïve blockchain

A really naïve blockchain is built, following the comprehensible tutorial here.

Cross-node communication and consensus are supported. However, a node has to add peers and request for peers' information completely manually. No broadcast mechanism. Besides, a node needs all its peers' full chain to resolve conflicts, which contains huge redundancy.