feat(local-testing): update debug build to enable local testing

This commit changes the `debug` build to enable testing from the local system.

All banner resources are loaded locally.
The banner configuration is loaded from a local `demo.json` file (added).

This is achieved by added [`grunt-preprocess`](https://github.com/jsoverson/grunt-preprocess)
which enables conditionals in the source file which is needed to replace
remote calls to `dothiv-registry.appspot.com`:

    // @ifdef DEBUG
    request.open("GET", "demo.json", true);
    // @endif
    // @ifndef DEBUG
    request.open("POST", "https://dothiv-registry.appspot.com/c?from=iframe&domain=" + document.domain, true);
    // @endif
fix(preprocess): let clean remove processed js files
Merge branch 'coderbyheart-local-testing' into coderbyheart-mobile-banner

Conflicts:
	Gruntfile.js