Fix responses of aborted xhr requests when user closes dialog or browses away from RP.

* Add an abort method to the XHR object.
* Keep track of all outstanding XHR requests. In the window.onbeforeunload handler, abort all outstanding requests.
** errors no longer logged on the KPIs when the user closes the dialog
** onlogout no longer called in the communication_iframe for aborted XHR requests
** onlogout called in the communication_iframe for all other XHR errors.

FWIW - changes to micrajax were pushed to origin repo - https://github.com/stomlinson/micrajax, https://github.com/stomlinson/micrajax/commit/2364453e387b914d556c53de8ab01188e067146f

fixes #2423
fixes #3618
fixes #3619
update incorrect test label
Use low level dom event binding functions to bind the beforeunload handler to window.
Merge pull request #3620 from mozilla/issues_2423_3618_xhr_abort_unload

Fix responses of aborted xhr requests when user closes dialog or browses away from RP.