Back on the road

Changes:

• Add Sphinx project and move current docs to it
• Add pytest support
• Support django up to 3.0.x
• Fix chainedfk and chainedm2m not being defined (closes #253)
• Add multichained selects to Location example
• Force to load select2 js first
• Fixed bug with missing "renderer" parameter in widgets.py render methods
• Fix renderer parameter in older django versions
• Add link to PyPI and installation instructions