grunt-modernizr and other changes

* added grunt-modernizr (with config coming from `modernizr.json`)
* added grunt-embed (to embed modernizr inline during build)
* removed html5 shiv, as this is done by modernizr
* got rid of es5 shim bower component; replaced with vendor script, because of the usemin issue with conditional comments... previously our build task was stripping out the conditional comments, meaning all browsers were downloading this script. we can change this back when usemin v2.0 comes out.
* refactored the prompts code – now using a features checklist like generator-webapp
* fixed bug where the extras-foot-1 SSI was inserted at the top of body