Load cache reload iframe as soon as we have a url

We should not wait for onload to fire because it might never fire.
Instead, we run the cache reload code as soon as we have a url, and
allow it to reload the user's browser cache.
get required version from query string.  this allows boomerang-reload.html to be cached forever
Don't save or report bw results if testing didn't complete before user navigates off page.

If a user navigates away from a page on which bandwidth is being tested
(i.e. before the file download probes are finished), the accuracy of
the results vary widely.

Therefore, don't save or report bw results if testing didn't complete
before user navigates off page.

A side-effect of this change is that bw testing will run on subsequent
pages until is finishes (which only requires that a user remain on a
page for 15s, so valid data should come quickly).
Updated comments to better reflect recent changes
Merge branch 'bw-calc-accuracy'