Updated draft of the URI scheme

 Please find it attached.

I ended up making quite a number of small changes - sorry about that.

Of note:

    * I decided to punish myself, and went through looking for all
      "must" & "should", and assessed whether they should be MUST/SHOULD
      or replaced with other text
    * I fixed the issues that Alexey raised
          o Biggest change here is the definition of "context default"
            and its use in the definition of several of the properties.
    * Caught a few typos here and there
    * I few places where I did rewordings to make the text clearer.

Let me know if you have any additional feedback.

-Eric.