Better step selection and redrawing upon resizing.

Same as:
https://github.com/bartaz/impress.js/commit/950bc935e6c08890930cc8432b3609ec72144374
Fix 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.
Merge pull request #1 from fmjrey/impress-compatibility

Impress compatibility