BrooklynGarbageCollector: conditionally do system.gc

- Exposes "brooklyn.gc.doSystemGc" config for whether to do
  System.gc periodically as well.
- Renames gc() to gcTasks() and adds javadoc for it
Add more to StringsTest.testSizeString
Merge pull request #598 from aledsage/fix/ahgittin-misc

ahgittin misc: incorporate review comments