Fixes #14. We now have documentation on creating a custom WorkerFactory.

Change-Id: Iaf4b0394c8cb02856116b589600d01d68736761e
Update the rest of the examples to use shared_ptr's of WorkerFactories.

Change-Id: Iec2874103791b9c8ee6818edc45b16b3a171097b
Merge branch 'derived_worker_factory'

Change-Id: I03db3e6937b29fb0f6e96a000c801ee6f381b8e6