v0.8 - Metrics listeners and new patterns

This is a fairly large new release of Pushy that makes some breaking API changes. The most important ones:

• Instead of constructing ApnsClient instances directly, we now use a builder pattern. To construct a new ApnsClient, use the ApnsClientBuilder class.
• ApnsClient is no longer a generic class; instead, the ApnsClient#sendNotification method is generic, so clients can now handle a wider variety of push notification types.
• We've removed support for custom flush thresholds, which turned out to be a little too complex for the benefit they were providing (and we expect an upstream implementation of the same feature shortly).

We've also included lots of non-breaking changes, too!

• We've publicized our own metrics listener that reports metrics using the Dropwizard Metrics library.
• We've publicized our mock APNs server to facilitate integration testing and benchmarking.
• We've added benchmarks using JMH.
• Upstream server errors are now treated as temporary write failures instead of permanent notification rejections.

For a complete list of changes, please see the v0.8 milestone.