Make XWiki core artifacts usable from an OSGi environment

There are several steps related to this:

• Add OSGi manifest information
• Add a generic OSGi Activator that registers XWiki components as OSGi services based on XWiki's annotations
• OSGi-ify third party dependencies that are not already OSGi-ified and make them available from the XWiki remote maven repository
• Provide 2 OSGi distributions for the XWiki rendering (since the rendering is made to be usable outside of XWiki):
  • an OBR XML file pointing to the XWiki remote repo for finding the OSGi-ified artifacts (alternatively use Nexus to wrap the XWiki remote repo and make it an OBR repo)
  • a zip file containing all bundles
• Documentation on xwiki.org on how to use the XWiki Rendering engine from an OSGi environment