Upgrade to metrics 3.0.1 to be compatible with dropwizard-metrics 0.7.0

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