Rename package to org.coursera.metrics.datadog

To match style from
https://github.com/dropwizard/metrics/tree/v3.0.2/metrics-graphite
Updated metrics-core to 3.0.2

Refactored DatadogReporter for metrics-core 3.0.2
Refactored MetricNameFormatter and DefaultMetricNameFormatter
Updated/Refactored DatadogReporterTest for mockito
Updated/Refactored Transport/HttpTransport
Removed MockTransport
Updated README