Impress compatibility

Two commits to improve compatibility with regular impress:
- reproduced minor upstream change bartaz/impress.js@950bc93
- fixed no initialization when impress.js is loaded in head tag. When a presentation loads impress.js in the head tag, the body element is not yet ready for consumption, making current logic fail to load supporting scripts. The fix is to target the head tag when body is not ready.

Before committing this PR you may want to check the second one (#2) that builds upon it for adding Strut (http://strut.io/) compatibility.