Make canvas offset determination more robust

Use jQuery offset approach (see http://stackoverflow.com/questions/13789879/getting-elements-global-document-coordinates-in-dart-aka-some-offset) - targeted for M6 easy enough to implement here.
todo to remove custom logic once it is supported
test.css for dynamic position canvas

added for testing proper canvas offset detection
add gameContainer div
Test resize and proper offset calcs

Added resize behavior to maintain 100% canvas constrained by defined aspect ratio.  Used withinCanvas dx/dy to trigger clamp[X|Y] logging.

With original canvasX/Y logic in game_loop_html this test would fail to determine proper withinCanvas and clampX/Y values.
brought latest master game_loop_html
Merge pull request #34 from alexgann/patch-1

Make canvas offset determination more robust