MIDI spec updates, as per telecon

I have updated the Web MIDI API specification, as per the discussion in
today's teleconference, with one addition.

The changeset is here: https://dvcs.w3.org/hg/audio/rev/f4727ce84474.

The updated spec is here:
https://dvcs.w3.org/hg/audio/raw-file/tip/midi/specification.html.

I made the following edits:

   - I expanded the overview section, making it clear that this API is not
   intended to cover semantic controls through MIDI (i.e. a solution to the
   web of things problem), and also that this API is not concerned with
   Standard MIDI Files or General MIDI - that is, that it is concerned with
   input and output, not "playback" per se.
   - I greatly expanded the introduction section to provide a more
   table-of-contents style overview of the API, and also to describe in more
   detail how the API is intended to function.
   - I essentially rewrote the Security and Privacy considerations section
   to describe the fingerprinting and access concerns in more detail, and also
   (per conversation in telecon) to explicitly leave the model open.  These
   three edits should resolve bug 19187.
   - I changed the sendMessage() method back to my suggested
   three-parameter form, and explicitly excluded sysex from sendMessage().  I
   expect further discussion on this point, but for our FPWD, I wanted to have
   it this way as I was brainstorming the security and privacy constraints,
   and I think it may be possible to use sysex as the "needs user permission"
   switch - that is, to require user permission ONLY in order to send/receive
   sysex.  I'm not positive this will be enough, but it will be easier to
   change it back than it would be to break variadic usage later.  ref: Bug
   18764.
   - I explicitly made timestamps in MIDIMessage allowed to be set to zero,
   with the semantic of "send now", as per Bug 18760.
   - I added IDs to several elements in order to provide forward links, and
   I expanded some of the IDL constructs to better show the descriptions of
   individual method parameters or members.  (no substantive changes.)

-Chris