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`
in fact libraries is often null due to the `CatalogItemAbstractDto` constructor methods.  i think we should remove the `@Nonnull` and allow null simply to mean no special libraries defined.