Improve Context id generation and unit tests
Add unit test to ensure an Async only generates one id
Rearrange default _process_results function
Make Context call persist after task insertion

First pass at making Context call persist after inserting its tasks.  I
think this isn't quiet correct though.  Each Async needs told it should
be firing the Context's "check complete" function after running.
Probably this can be hooked into the Async's callback logic.
Fix tests_stubs tests

Minor fixes for api changes from a while ago.
Since the "tests_stubs" directory starts with "tests_", it isn't/wasn't
run automatically by nosetests when the code was changed.
Add "enable_retries" to run_queue()
Make sure to increment the eta

We are ignoring the eta, but when app engine postpones a task for
a retry, it verifies the eta has increased.
More comprehensive test for retries

Ensure the retry parameter is tracked separately for tasks.
No new failures, but this better verifies that retries with the
queue runner are working.
Fix bool docstrings
Split result processing logic out from run_job

Move result processing logic into its own function to clean up code and
make testing more reliable.
Add event hook for context check complete call

Add an event hook that runs after run_job in order to trigger a Context
completion check.
Readd param description to docstrings
Add param docstrings for the two new functions
Add the containing Context.id to each contained Async

When an Async is added to a Context, add the Context's id to the Async.
The Context id is useful for reporting and grouping.
Merge pull request #124 from ericolson-wf/add_retries_to_test_runner

Add retries to test runner
Correct _parse_yaml_config's handling of empty string

Correct _parse_yaml_config's handling of empty string to treat empty string
as an empty config object.  This will result in the default config being
returned if the empty string is passed in.  None will result in the
loading of the default config yaml.
Add a simple example of using Context callbacks
Update Async to support specifying a context checker

Add an internal '_context_checker' option to Async to facilitate Context
completion checking.  This is meant as an internal parameter only for
internal use.
Add unit test for correct _context_checker encoding
Pass whole Async to context checker

Instead of passing only the Async options to the context complete
checker, pass the entire Async.  This should enable the context checker
to take advantage of any methods defined on Async, particularly
customized Asyncs.