Add excanvas.js and excanvas.min.js from current, 1.0.8r1250, jqPlot release

While these files could be obtained by the user from a different location, they are provided in the 1.0.8 release of jqPlot (https://bitbucket.org/cleonello/jqplot/downloads/jquery.jqplot.1.0.8r1250.zip) and referenced in the instructions/examples (http://www.jqplot.com/docs/files/usage-txt.html). Thus, this is where most users would be expecting to find them.  They probably don't notice that the files are missing because they don't test in older versions of IE.