Update CONTRIBUTING.rst

Thanks to @mauricioabreu
Clear up testing directions

tox should be used by default
Break out requirement files

* dev file for development requirements
* test file for test environment requirements
Allow configurable TimeStampModel

This commit adds the possibility to set a TimeStamped model via
settings.
ORGS_TIMESTAMPED_MODEL is required to run otherwise it will raise a
ImproperlyConfigured exception.
Account for missing return class

`get_model` will return None if the app is not installed.

Closes gh-41
Merge pull request #42 from mauricioabreu/configurable-timestamp-model

Allow configurable TimeStampModel

Closes gh-36
Provide a django_extensions default

This allows for smoother backwards compatibility
Minor verbiage cleanup [ci skip]
Update the slug field so it is configurable

This is not wholly configurable in so far as using the `populate_from`
keyword argument with `django.db.models.fields.SlugField` will result an
error. However it should be cofigurable between multiple AutoSlugField
implementations. Django-extensions and Django-autoslug both use this
arguments.

Closes gh-35
Version bump