Swap out some AtomicLongs for LongAdders.

This allows us to trade a bit of memory for a huge improvement in the performance of contended counters and meters. Striped64 and LongAdder are from the JSR166e project led by Doug Lea and are in the public domain. They're intended to be included in JDK8, so Metrics should remove these copied implementations once it moves to JDK8.
Make map-building in MetricRegistry overrideable.

Closes #373.
Move all the tests out of test packages.

No one seems to want to do this, including my own IDE.
Update Maven module names and descriptions.
Make metrics-json optionally depend on health checks.
Ported @romseygeek's removeMatching.

Not only did I give bad advice on #377, but I also moved the tests around underneath it, which makes it unmergeable. So instead of making him rebase it, I'm doing it myself.

Closes #377.
Update release notes and contributors.
Fix listener notification in MetricRegistry#removeMatching.
Mention the JMX reporter in Getting Started.