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

Made updates to the BV version of the `metrics-datadog` code.

Updated metrics-core to 3.0.1 and jackson-core to 2.3.2
    Rename package to com.codahale.metrics.datadog
    Follows same pattern as
    https://github.com/dropwizard/metrics/blob/v3.0.1/metrics-graphite/src/main/
    Changed package name to com.codahale.metrics.datadog
    Refactored DatadogReporter for metrics-core 3.0.1
    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