Adds global params section.

Ernables a global parameters section per the thread in:
https://github.com/Hipo/Django-API-Playground/issues/5 

A parameters block may be specified directly at the schema root which will result in a data form being displayed at the top of the page. These elements are then applied to all the API calls on the page.

Example:
	"parameters": [{
		"name": "apikey",
		"type": "string",
		"is_required": True,
		"description": "apikey"
	}, {
		"name": "format",
		"type": "select",
		"choices": [["json", "json"], ["jsonp", "jsonp"], ["xml", "xml"], ],
	}]

This can admittedly use some design help.