Add progress and loading attributes; includes docs, demo, and test.
Address comments; improve reliability of progress test.

 * changed single-quoted attributes to double quoted
 * use a slower network request to more reliably assert about the request
   while it is loading
 * add a core-range to the progress test in order to catch the assignment to
   read-only property bug
minor: set progress to null on the prototype
minor: document the progressProxy object
Merge pull request #27 from rictic/progress

Add progress and loading attributes; includes docs, demo, and test.