Support MPEG-DASH using dash.js

[mattfawcett]

I added support for MPEG-DASH streaming.

To use it, you include a 'mpdv' file in your setMedia call as well as adding mpdv to the 'supplied' string. Mpeg Dash will be used if the browser supports it (MediaSource extensions are available), and the dash.js library is available.

There is an existing feature request for this (issue #304 )