Filter which sheets get parsed

This PR adds a global respondJsSheetFilter function that you can set to tell respond.js which sheets to parse. It defaults to ```return true;``` which will parse all of them.

Otherwise you can define it however you need, say with a regex:
```
window.respondJsSheetFilter = function(req) {
    return /some_regex/.test(req.href);
}
```
To only parse sheets that match a given pattern. Just define respondJsSheetFilter before you load respond.js.

As for why I added this... While rendering some maps with VML overlays for IE8, for some reason having respond.js loaded caused the overlays to not render. I'm no expert in VML so this probably isn't the most correct solution, but I worked around the problem by having respond.js only parse the css files with media queries in them.