Handle when an activity type does not exist

[ggreg]

In this case, the executor has to create the activity type and let the replay behavior try to schedule the activity again.

This allows to remove the .get_or_create() calls from ActivityTask.schedule() and to only query SWF when needed. It also permit to run all the tests without AWS API keys.

[ggreg]

It should be handled as a case of #5 when the activity could not be scheduled because the activity type is not registered i.e. does not exist.

[ggreg]

There is still an issue with simple-workflow that connects to the SWF API whenever it instanciates a model.

[ggreg]

The rationale for this issue is:

1. Avoid to call .get_or_create() when scheduling a task because it always at least involves one call to the SWF API and add a significant latency if many tasks are scheduled.
2. Do not require AWS credentials to launch the tests.

1. is solved. But not 2.. We could solve 2. by mocking the connection to AWS.