Fix path for inclusion of Respond.js file

These are two changes in single line edit:
- fix path for including Respond.js. Path is bild with helper now to
avoid usig wrong path translation based on hardcoded relatvie path
- convert code to use helper for JavaScript script tag:
http://getkirby.com/docs/helpers
Merge pull request #1 from peterblazejewicz/RespondScriptFix

Fix path for inclusion of Respond.js file