[#25] Check that the browser supports Canvas explicitly, instead of testing for IE

What motivated me is that $.browser was deprecated in JQuery 1.3, and removed
in 1.9. Thsi new test is faster and better.

Check
http://stackoverflow.com/questions/2745432/best-way-to-detect-that-html5-canvas-is-not-supported
for more info.
Merge pull request #26 from openspending/25-deprecated-jquery-browser

[#25] Check that the browser supports Canvas explicitly, instead of testing for IE