Add limits example

This is not so much an example as a testbed for testing queue runtime
characteristics as related to task runtime.
Allow AutoContexts to be created from context.new()

This adds a batch_size kwarg to context.new which results in an
AutoContext being registered instead of a regular Context. Aside from
being more convenient, this allows the AutoContext behavior to be
abstracted from the client. This helps in the event that we decide to
bring AutoContext's batching functionality into Context, removing the
need for the subclass.
Use AutoContext proper noun in docstrings to be more clear
Merge pull request #139 from tylertreat/auto_add

Allow AutoContexts to be created from context.new()
Merge pull request #138 from tannermiller-wf/limits_tester

Add limits example