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
`Datadog` class was meant to mimic the style here - https://github.com/dropwizard/metrics/tree/v3.0.2/metrics-graphite/src/main/java/com/codahale/metrics/graphite

If you look at 3.1.0, you can see it's been updated to be organized differently (https://github.com/dropwizard/metrics/tree/v3.1.0/metrics-graphite/src/main/java/com/codahale/metrics/graphite). I can go back to the `Transport` class from before if you feel that's better. Would you prefer the `Transport` class or how it's done in `3.1.0`.