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
Hi Chris! I see the `Graphite` class. I think it makes more sense in that context i.e. keeping a long-lived socket connection open to the (potentially) local graphite server.  In our case, we're making a new HTTP request to Datadog every reporting interval, so I see less of a need to . In both cases, I don't see the need to expose the transport implementation to the client.  

I know it's a bit of work to switch back. Do you mind? I like the `Datadog` class over the older `HttpTransport` in that it encapsulates the building of the Datadog series request. To that end, perhaps we can use the `Datadog` class to `prepare()` a "one time" `DatadogSeriesRequest` to which we add gauge/counters and then `send`.