Added Mask blend mode
Changed HtmlExternal to bind functions to the container

This is more consistent with the way it works for Flash, and
also allows works when using multiple Flambe instances.
flambe.embed can take a callback that is fired when embedding is complete
CanvasGraphics#clear now uses clearRect instead of fillRect

Using fillRect could break transparency.
Initially add canvas with no width/height

This is because some browsers (e.g. chrome) give a default width/height
that can cause it to become larger than its parent, breaking stage size
detection.