Rearranged some methods.
Canvas now uses 100% width/height in CSS and adjusts to
clientWidth/clientHeight in _render.
Fixed an edge case when attempting to position the content off the edge of
the screen and then checking against obstacles.
If the desired direction would always be outside of the viewport, then
don't bother checking any obstacles.
Broke up jquery.tutorialOverlay into jquery.rectUtils,
jquery.tutorialOverlayArrow, and jquery.tutorialOverlayTip.