Visual highlight; Docs synch/update; Build

* Add a callback that allows the chart's visual highlight to follow the sound.
* Synch index.html with README wrt browser support.
* Add a note about TDD to the Development section in README.
* Start to make Makefile more portable (not tested yet).
* audio_context_getter() abstracts cross-browser stuff.
* Move away from '(expect...' in tests to 'expect(' (more sane).