Upgrade to metrics 3.0.2 to be compatible with dropwizard-metrics 0.7.X

Sorry for the new pull request. I left out some fixes in the tests.

Wanted to update this reporter to metrics-core 3.0.1 to be compatible with dropwizard-metrics 0.7.0. The benefit of that integration point is that you get the benefit of adding the reporter and setting the parameters through a configuration file for a dropwizard application. I plan on sending another pull request for the dropwizard-metrics integration if Coursera wants to manage that too.

Rename package to com.codahale.metrics.datadog
To match style from
https://github.com/dropwizard/metrics/tree/v3.0.1/metrics-graphite
Updated metrics-core to 3.0.1 and jackson-core to 2.3.2
Follows same pattern as
https://github.com/dropwizard/metrics/blob/v3.0.1/metrics-graphite/src/main/java/com/codahale/metrics/graphite/GraphiteReporter.java
Refactored DatadogReporter for metrics-core 3.0.1
Refactored MetricNameFormatter and DefaultMetricNameFormatter
Updated/Refactored DatadogReporterTest
Added Datadog class for creating JSON/posting http request
Updated README
This is pretty verbose and could be difficult to maintain going forward. Perhaps we could do this iteratively? E.g.
```
Expansion[] timerExpansions = Expansions.TIMER_EXPANSIONS;
double[] durations = [snapshot.getMax(), snapshot.getMean(), ...];
for (int i = 0; i < durations.length; i++) {
  datadog.addGauge(maybeExpand(timerExpansions[i], name),
                   format(convertDuration(durations[i])),
                   timestamp,
                   host,
                   tags);
}```