fix($httpBackend): 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`.
See http://pieisgood.org/test/script-link-events/

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
chore(style): fix missing indentation in httpBackend from a3172a2
docs(CHANGELOG): add release notes for 1.2.2 consciousness-inertia
chore(release): cut 1.2.2 consciousness-inertia release
chore(release): start 1.2.3 unicorn-zapper iteration
chore: update copyright year in file headers
chore(docs): remove Disqus comments

We don't actively moderate these comments, and they range from
out of date, to inflammatory, to spam. Going forward, improvements
to the docs should be done via a PR, and questions should go on
StackOverflow where they can be curated and kept up to date by
AngularJS developers who help out there.
fix(input): Support form auto complete on modern browser

Although modern browser support the "input" event, they still only fire
the "change" event when they auto complete form elements
other than the currently selected one.

Related to #1460
fix($animate): ensure blocked keyframe animations are unblocked before the DOM operation

Closes #5106
docs(CONTRIBUTING): consolidated submitting PRs sections
docs(CONTRIBUTING): highlight what makes a good issue submission
docs(CONTRIBUTING): fix typo
fix(ngAnimate): ensure animations are disabled upon bootstrap for structrual animations

Closes #5130
docs(CONTRIBUTING): add link to github-pr-helper
chore: add script for updating bower repos
chore(travis): let's give BrowserStack a try

Switch the build to use BrowserStack instead of SauceLabs.

This also adds IE11 to our build.
chore(travis): add some more info for BrowserStack sessions
refactor(angular.js): improve trim performance

According to Flagrant Badassery's blog
http://blog.stevenlevithan.com/archives/faster-trim-javascript
and this comparison http://jsperf.com/trim-function, this trim method is faster.

Closes #4406
docs(guide/scope): access the current element's scope in the console.

Closes #4884
docs(CONTRIBUTING): fix broken link to GitHub PR Helper

Closes #5134