Shift initialisation into recordmp3.js and attach event listener

Rather than using window.onload in the HTML, attach an event listener in
the JS so that it's more self-contained.  Also shift startUserMedia.
Build interface in JS, support multiple instances
Encapsulate __log function
Make record button toggle recording

This means that you can click the record button again to stop recording,
rather than having to click the stop button.
Support callbacks and format selection via data-attributes

The recorder element should specify a data-callback="functionName"
attribute, and can also specify data-format="mp3" to receive an audio/mp3
blob rather than audio/wav
Correct logging from uploadAudio callback in demo.
Add playback support + play/pause button
Convert indentation to consistent 2 spaces
Move callback processing to new "save" button.
Basic visual styling

Including shifting the Bootstrap imports into the demo HTML, so that the
stylesheet can be included safely in other sites.
Use icon font for buttons

Entypo pictograms by Daniel Bruce — www.entypo.com
Add VU meter in record button (when recording)

To give an indication to the user that something is happening.
Whitespace - fix indentation
Clean up unused/commented out code