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
I don't think we need to expose the `Datadog` class to end users. The only input parameter is apiKey, which can be configured in the reporter's builder via a `with` method. `Datadog` is not thread-safe as implemented and users may accidentally reuse it in multiple `DatadogReporter`s.  This would open up the possibility for nasty race conditions when both reporters run concurrency.  Also, none of the public methods on `Datadog` should be directly accessed by end users.

Per my comment above, I'd recommend implementing "single use" transport class (as done in the previous version of the reporter) that sends the report to Datadog once and is then destroyed.  Re-using the `OutputStream` between reporting intervals is a bit more efficient, but the reporter is run very infrequently and it seems reasonable to build a new OutputStream for each send. I can be swayed on this.