fix(): only IE8 and below can't use `script.onload` for JSONP

IE8, IE9 and IE10 can use `script.onreadystate` so up till now we have been using this
if the sniffer says we are on IE.
But IE11 now does not support `script.onreadystate` and only supports the more standard
`script.onload` and `script.onerror`.
IE9 and IE10 do support `script.onload` and `script.onerror`. So now we only test whether
we are on IE8 or earlier before using `script.onreadystate`.

jQuery just uses all these handlers at once and hopes for the best, but since IE9 and IE10
support both sets of handlers, this could cause the handlers to be run more than once.

jQuery also notes that there is a potential memory leak in IE unless we remove the handlers
from the script object once they are run.  So we are doing this too, now.

Closes #4523
Closes #4527
Closes #4922