add mocha-phantomjs, chai, downgrade mocha

regarding temporary downgrade of Mocha from 1.9.x
to 1.8.x see:

https://github.com/visionmedia/mocha/issues/770
https://github.com/metaskills/mocha-phantomjs/issues/58
browser tests: generate embeddable-test-mocha.html

Generate variation of embeddable.html for testing in browser with
mocha or mocha-mocha-phantomjs

To run headless phantomjs must be installed.

./node_modules/mocha-phantomjs/bin/mocha-phantomjs -R dot public/embeddable-test-mocha.html
update deprecated form in jade template

Removes deprecation warning during build.

see:
https://github.com/visionmedia/jade/pull/1036
Deprecate implicit text-only for script and style
Makefile: remove extra resources copied to public/lab

Also cleanup and comments.

There is a new task: clean-public.

Running:

  make clean-public; make public

Will be faster and in some cases will be sufficient to replace:

  make clean; make