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
* 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