Updated metrics-core to 3.0.2 and jackson-core to 2.3.2

Follows same pattern as
https://github.com/dropwizard/metrics/tree/v3.0.2/metrics-graphite/src/main/java/com/codahale/metrics/graphite
Changed package name to com.codahale.metrics.datadog
Refactored DatadogReporter for metrics-core 3.0.2
Swapped out httpclient for fluent-hc in pom.xml
Added Datadog class and removed HttpTransport/Transport
Updated/Refactored DatadogReporterTest to use more mockito
Updated README