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.
Slf4j 1.6 is binary compatible with 1.7 - change OSGi import

The OSGi import for SLF4J is defaulted to 1.7-2.0 by Maven. Since 1.7 is
binary compatible with 1.6, users of the Metrics API should not be required
to use 1.7+. Modify the import to indicate use of 1.6+ is acceptable.
Servlet API 2.5+ is acceptable at runtime

The build uses the Servlet API 3+ due to its dependency on Jetty 8.
However, there is no capability used by metrics that is dependent on
Servlet 3. Modify the build to allow the OSGi bundles created to be used
in a Servlet 2.5+ environment.
Added HttpClient metric naming strategies
Added doc note for http metric naming strategies
Move Metrics to com.codahale namespace.

This should allow for an easier upgrade path by reducing Maven and classpath conflicts.

Still keeping licensing and copyright info.
Add gauge for pending jobs in InstrumentedQueuedThreadPool.
Added metrics-jetty9 w/ InstrumentedConnector.
Added InstrumentedQueuedThreadPool.
Added InstrumentedHandler.
Upgraded to Jetty 8.1.10.
Fix Maven packaging for metrics-jetty9.