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.
Add YourKit links.
[maven-release-plugin] prepare release v3.0.0-BETA2
[maven-release-plugin] prepare for next development iteration
Bump the doc version.
Update release notes.
Merge pull request #380 from vivosys/slf4j-import-range

Allow older versions of slf4j and servlet API in OSGi imports
Upgrade to Logback 1.0.11.
Merged latest and moved http client metric name strategy classes to new package structure
Open up metrics-httpclient a bit.

Closes #384.