Adding support for Django RestFramework

[enzofrnt]

Related to: #140

Hi, I've just upgraded the plugin to fully support the Django Rest Framework.

Modifications include:

• The old code has been retained, with minor upgrades.
  • Adding to django_eventstream.utils a get_cors_from_settings method
• Rearranged the static folder organization.
• Added a viewset for Server-Sent Events (SSE) that is capable of redirecting to the correct renderer with the appropriate response.
• Introduced a renderer for SSE streams. This renderer is equipped to handle SSE streams with the correct response headers and the right content type.
• Implemented a renderer for the SSE browsable API:
  • Added browsable-api-eventstream.html, a template file that is an edited version of the base.html file from the Django Rest Framework.
  • Added browsable-api-eventstream.js, a JavaScript script that provides all the necessary functionality for the browsable-api-eventstream.html template to correctly display the SSE stream, using reconnecting-eventsource.js.
  • Added browsable-api-eventstream.css to supply the CSS for the browsable-api-eventstream.html template.
• Updated DOCs
• Adding possibility to install with or without Django Rest Framework. Check __init__.py files.
• Adding a fully functional example chat-restframework

The Browsable API is fully usable, as shown here:

The system will automatically return an SSE Stream when called by a request header that contains Accept: text/event-stream.