Adding get_current_context() method
Add MessageIterator for pulling work off a pull queue.

Add a MessageIterator which pulls messages off a specified pull queue in
specified batch amounts. It exposes an iterator to go over the messages and
send out the payload from the messages
removing further reference to _executing_async in get_current_context
Remove get_messages call as the MessageIterator does what's needed.

get_messages is no longer needed.
Fixed typos in the comments and the Exception. Also fixed if condition to check registry
Add an async_intro module with the basic intro async example.
Merge pull request #25 from beaulyddon-wf/message_iterator

Add MessageIterator for pulling work off a pull queue.
Merge pull request #24 from tannermiller-wf/get_current_context

Adding get_current_context() method
Add a basic context_intro module to examples.
Add a callback module to exmamples for showing the different callback styles

This has 3 examples of the different ways to wire up callbacks.
Add a simple_workflow module to examples.

This example shows a simple flow of chaining Asyncs together.
Add complex_workflow module to the examples.
Update the example app to point to the broken out example modules
Cleanup the complex workflow example.
Merge pull request #26 from beaulyddon-wf/cleanup_examples

Cleanup examples