Don't check for full screen support at load time

Detection of full screeen support requires the document to have a body. This is not the case if ol3 is loaded in the head.  Therefore, defer the test for full screen support to when it is needed.

Refs #1767 and [the mailing list](https://groups.google.com/d/msg/ol3-dev/SEu5Js8OurU/MQgZ8WM4ER8J).