Basic demo: set HTML5 doctype

This keeps IE8 out of quirks mode, which breaks things like 
getWindowSize…
Viewer: refactor message code into showMessage/hideMessage methods

This avoids the need to deal with classes, and explicitly makes it
reusable for other needs
Refactor OpenSeadragon.makeCenteredNode

* Use CSS display tables for vertical centering (tested back to IE8)
* Use the DOM instead of string concatenation
* Remove redundant styles ($.makeNeutralElement sets the same values
  for margin, padding & border)
* Return the outer wrapper element to ease DOM addition & removal