Add feature detection for CSS Shapes

We were discussing about split up CSS Exclusions tests to 'wrap-flow' and 'shapes' in the #786 pull request. This request contains the Shapes part. Currently, only WebKit based browsers support CSS Shapes (http://www.w3.org/TR/css3-exclusions/#shapes), e.g. Chrome Canary or Safari Nightly. 

Using Modernizr feature detection for Shapes demos just would be a convenient solution like in the case of wrap-flow. I put together the minimal test case that is required for Shapes. I hope you like it.