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
@tianx2 in the last push, I have moved serialization out of HttpTransport:

https://github.com/coursera/metrics-datadog/blob/2.x-maintenance/src/main/java/com/yammer/metrics/reporting/serializer/Serializer.java

I have also modified the Transport interface to look like:
https://github.com/coursera/metrics-datadog/blob/2.x-maintenance/src/main/java/com/yammer/metrics/reporting/Transport.java

So this is how HttpTransport looks like now:

https://github.com/coursera/metrics-datadog/blob/2.x-maintenance/src/main/java/com/yammer/metrics/reporting/HttpTransport.java

HttpTransport now uses JsonSerializer. 

I intend to submit the same pull request for this branch as well. Those changes has been merged to the 2.x brach. You can look at the commit here: https://github.com/coursera/metrics-datadog/commit/7419a2336ff37900511865f915d379ff80341d57