Fixes #17. Make it possible to use a derived WorkerFactory.

Change-Id: Ib56df2bc24131ac41715458b8db168bd46ada457
Fix some logical-not-parentheses warnings.

Change-Id: Ibfaf2faf2b6c094a495d02f488e55b9f5396027a
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: Ibdfc0fa6f66a986a99a18276f87154445a258799