Initial OSGI work

Towards BROOKLYN-13.
* Adds @ahgittin's initial work with Felix
* Updates catalog.xml to load and store bundle URLs under a `<libraries>` element.
* Adds helpers for loading classes from bundles to `BrooklynEntityClassResolver`
Longer term, would be nice to have a Version class that wraps the string - it would support comparisons etc.
Ideally we'd also use some third part library that correctly implements semantic versioning.
Alternatively, if we used OSGi versioning (different semantics as I recall, such as `1.20 < 1.3`) then maybe Felix has an appropriate comparator.