initializing dynamically added canvas when IE8/excanvas methods are avai...

In working with AngularJS, ChartJS, and your angular-chart.js, I ran into problems in IE8 even when [excanvas](https://code.google.com/p/explorercanvas/) was on the page. This was due to the fact that the canvas elements are added after DOM load and need initialization (in IE) ([docs](https://code.google.com/p/explorercanvas/wiki/Instructions)). I also ran into some problems with ES5 usage in the directive. I worked through the latter with the addition of [ES5-Shims](https://github.com/es-shims/es5-shim), and now submit to you a request to pull in code which will initialize the canvas elements in IE.

Using ES5-Shims, excanvas, and this modification to your directive I am now confidently using the directive in IE8.

Thanks for your consideration.