Allow nesting of html spec runner

Jasmine spec runner builder is currently hardcoded to seek assets from './' that forces us to situate the `outfile` at the root of project. This PR makes builder respect nesting of HTML runner and fix assets path accordingly.

I.e. runner put into `spec/runner.html` would generate `../` prefix for every `<script>` and `<style>` tag. Tests included :yum: