Add Metrics API and RESTAPI class

[cgcardona]

• Add MetricsAPI class
• Add RESTAPI class
• Add tests

Callable like this:

const avalanche: Avalanche = new Avalanche(ip, port, protocol, networkID);
const metrics: Metrics = avalanche.Metrics();
const main = async () => {
  let m = await metrics.getMetrics();
}

This addresses Issue 39.