Add NetworkMonitor class

[maraoz]

This PR adds a new class: NetworkMonitor.

The idea is to have a simpler interface than PeerManager to listen to bitcoin network events.

Simlpe usage example:

var config = {
  networkName: 'testnet',
  host: 'localhost',
  port: 18333
};

var nm = new NetworkMonitor.create(config);

// monitor incoming transactions to http://tpfaucet.appspot.com/ donation address
nm.incoming('msj42CCGruhRsFrGATiUuh25dtxYtnpbTx', function(tx) {
  console.log('Donation to tpfaucet! '+JSON.stringify(tx.getStandardizedObject()));
});

// connect to bitcoin network and start listening
nm.start();

This PR also adds some refactoring and convenience methods for other classes. Most importantly:

• Address.fromScriptSig, extracts an address from a transaction's scriptSig
• Transaction.getReceivingAddresses, returns a list of addresses that received coins in a given transaction
• Transaction.getSendingAddresses, returns a list of addresses that sent coins in a given transaction

[ryanxcharles]

Awesome. Will review soon.

[coveralls]

Coverage increased (+0.37%) when pulling c4de367 on maraoz:add/NetworkMonitor into b172248 on bitpay:master.

[ryanxcharles]

Pulling in due to offline discussion.