Annotator-store

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* Creating new annotations via annotator API now requires the user
to have the `annotator_store.add_annotation` permission.
(Previously any logged in user could create annotations.)
* Any annotation edits (create, update, delete) made via annotator API
now generate Django admin log entries.

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Initial release of `annotator_store` as a stand-alone Django application,
refactored out of the `Readux <https://github.com/emory-libraries/readux>`_
codebase.

* Support for custom Annotation model via **ANNOTATOR_ANNOTATION_MODEL**
setting and `annotator_store.models.BaseAnnotation` abstract model
* Configurable support for normal Django permissions *or* per-item
permissions via **django-guardian** (*NOTE* that per-item permissions
functionality is not fully tested or supported and should be
considered alpha quality)
* Annotation text and quote fields marked as optional for use in Django admin
* Supports both Python 3.x and Python 2.7
* Includes example templates with code for initializing an annotator.js
instance and connecting it to annotator_store as a backend.