[EME] Recent spec changes and bug updates

I made a number of spec changes over the last two weeks, including several
bug fixes, updating many of the algorithms, and various other improvements.
See https://dvcs.w3.org/hg/html-media/log for the full list of changes. I
also re-opened some bugs to deal with issues I found while editing the
spec. In most cases, there are simple suggested changes.

Fixed:

   - Bug 20622 - SessionID may be assigned asynchronously in
   MediaKeys.createSession
   - Bug 23618 - Drop "key" prefix from event names
   - Updated stack overview figure to match the text. (It is now displayed
   using SVG directly.)
   - Bug 19809 - Specify which portion of addKey() algorithm to run when
   updating license for a key
   - Removed two CORS-related check previously added for Bug 21203 - EME
   leaks information cross-origin - Remove CORS step from update().
   - Improved on previous fix and updated examples for Bug 19096 - Add
   'type' attribute to MediaKeyNeededEvent


Updated algorithms:

   - createSession()
   - update()
      - (Also renamed the parameter from |key| to |response| - I'm open to
      other suggestions.)
   - First Time a Key Reference is Encountered algorithm.
   - Encrypted Block Encountered


Reopened bugs and new comments:

   - https://www.w3.org/Bugs/Public/show_bug.cgi?id=20991#c7
   - https://www.w3.org/Bugs/Public/show_bug.cgi?id=21203#c29
   - https://www.w3.org/Bugs/Public/show_bug.cgi?id=19156#c6
   - https://www.w3.org/Bugs/Public/show_bug.cgi?id=21854#c23
   - https://www.w3.org/Bugs/Public/show_bug.cgi?id=17750#c16