Get spectrogram working again against recent bokeh

don't ignore y units when mapping from screen

accept ArrayBuffers for image_rgba again

initially working spectrogram with current bokeh, except line plots missing axes

ignore spectrogram.js

use Bokeh.Plotting to create the plots instead of manually

fix #303

remove spurios ctx save/restores

used typed array in simple index plot

add grunt-contrib-symlink and set up to build and copy spectrogram demo to build

remove spurious symlink to bokeh.js
update READE for spectrogram
Merge branch 'spectrogram'