WIP: P2P Peer Connections

Adds `bitcore.transport.Peer` and `bitcore.transport.Messages` allowing p2p connections with the bitcoin network.

Peer creation:
```javascript
var bitcore = require('bitcore');
var Peer = bitcore.transport.Peer;

// default port
var livenetPeer = new Peer('5.9.85.34');
var testnetPeer = new Peer('5.9.85.34', bitcore.testnet);

// custom port
var livenetPeer = new Peer('5.9.85.34', 8334);
var testnetPeer = new Peer('5.9.85.34', 18334, bitcore.testnet);

// use sock5 proxy (Tor)
var peer = new Peer('5.9.85.34', 8334).setProxy('localhost', 9050);
```


The interface works as follows:
```javascript
var peer = new bitcore.transport.Peer('5.9.85.34');

peer.on('ready', function() {
  // peer info
  console.log(peer.version, peer.subversion, peer.bestHeight);

  var message = new bitcore.transport.Messages.GetAddresses();
  peer.sendMessage(message);
});

// handle events
peer.on('inv', function(message) {
  // message.inventory[]
});
peer.on('tx', function(message) {
  // message.transaction
});
peer.on('addr', function(message) {
  // message.addresses[]
});

peer.connect();
```

# To do:
- [X] Add tests to messages.js
- [ ] Write docs
Can we put these on a different file? As per @ryanxcharles suggestion on https://github.com/bitpay/bitcore/issues/691