Add an example runner for easily running the examples.

The script takes in the url to run and optionally the sdk path.

Right now this only works for localhost. More robustness for hitting GAE
appspots coming later.
Update the way the url is handled.
Add docstrings to the example/runner.py module.
Add module docstring to example/runner.py
Merge branch 'master' into add_example_runner

* master:
  Fix test comment for test_async
  Have Async.start retry on TransientError
  Add basic error handling for start on Async.
  Fixed typos in the comments and the Exception. Also fixed if condition to check registry
  Remove get_messages call as the MessageIterator does what's needed.
  removing further reference to _executing_async in get_current_context
  Add MessageIterator for pulling work off a pull queue.
  Adding get_current_context() method
Merge pull request #28 from beaulyddon-wf/add_example_runner

Add an example runner for easily running the examples