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)