Recent changes in the Web MIDI API spec

Doug suggested I should capture all the recent changes in the Web MIDI API,
since I checked in a flurry of changes over the last week.

In short:

   - MIDIPorts now have separate device and connection states, and ports
   can be unplugged-but-open (and will then reopen when reconnected).  This is
   based on issues #79 <https://github.com/WebAudio/web-midi-api/issues/79>,
   issues #123 <https://github.com/WebAudio/web-midi-api/issues/123>.
   - There were some incorrect uses of DOMError that should have been
   DOMException (issue #90
   <https://github.com/WebAudio/web-midi-api/issues/90>).
   - It turns out MIDI Realtime messages can in fact occur in the middle of
   short messages, too. Accounted for this with issue #47
   <https://github.com/WebAudio/web-midi-api/issues/47>.
   - Added a clear() method on output ports to clear pending messages.
   Issue #102 <https://github.com/WebAudio/web-midi-api/issues/79>.
   - Added an open() and close() pair to better enable port management on
   exclusive-access systems, but with implicit opening to make simple
   scenarios work.  Issue #75
   <https://github.com/WebAudio/web-midi-api/issues/75>.
   - Miscellaneous Maplike fixups, and clarifications.


Full view of commits:
https://github.com/WebAudio/web-midi-api/commits/gh-pages.

I'd like to review the open issues list (
https://github.com/WebAudio/web-midi-api/commits/gh-pages) at the next
telecon.