Separate dev dependencies into extras package via setuptools

[campb303]

Setuptools supports installing situational dependencies as stated here: https://setuptools.readthedocs.io/en/latest/userguide/dependency_management.html?highlight=install_requires#optional-dependencies

We should separate out our docs and dev dependencies so that we only install what is needed when installing a package from pip.

Instal Type	Example	Packages Installed
Normal	pip install webqueue2-api	Everything needed for the API
Development	`pip install webqueue2-api[dev]	Everything needed for API + things for development (like pytet)
Docs	`pip install webqueue2-api[docs]	Everything needed for the API + things for docs (like mkdocs)

[campb303]

An example of this can be found in Flask's setup.py file here: https://github.com/pallets/flask/blob/1.1.x/setup.py#L61-L78

[campb303]

This was merged in #18 . Closing.