Cleaning up ThreadPool.

No need for separate pools, as Charger is out.
Add SystemStatus provider impl to try and break away from ApplicationStatusSource usage
add headers
NX-163: use extender model to register plugins/bundles (fixes #415)

 * New nexus-extender automatically registers plugin/bundles with Nexus as they're started
 * Embedded nexus-core and associated support jars as part of nexus-extender bundle
 * Bootstrap layer now consists of nexus-bootstrap, some logging/metric jars, and framework launcher
 * Webapp bootstrap mode launches OSGi framework + Nexus from inside Jetty/Tomcat/JavaEE
 * Jetty/OSGi bootstrap mode launches Nexus on top of an existing OSGi framework
 * Includes example Karaf assembly which can launch Nexus (see README for instructions)
deprecate [Abstract]ApplicationStatusSource
Merge pull request #434 from sonatype/system-status-provider

TRIVAL: System status provider
[NEXUS-6495] guard against NPE when rendering error page template
[NEXUS-6496] avoid NPE when handling redirect logic/logging
[NEXUS-6492] revise handling of target mapping when request path does not match pattern, improve logging
[NEXUS-6498] change warning to info, and log when we detect unlimited strength, vs. absence
[NEXUS-6491] omit stack traces in error page responses
Merge pull request #423 from sonatype/repo-threadpool-cleanup

Cleaning up ThreadPool et al
Merge remote-tracking branch 'origin/master' into timeline-kazuki