fix($anchorScroll): make sure scrolling takes place when `readyState === 'complete'`

When navigating to a page with a hash in the URL, Firefox and IE do their
native scrolling AFTER $anchorScroll, which cancels the applied `yOffset`.
Waiting for `document.readyState` to be 'complete' solves the problem in
Firefox.
(In IE, the issue still appears when navigating to a URL (with a hash),
but not when reloading the same URL.)

Note, that with or without this fix subsequent calls to `$anchorScroll()`
or changes to `$location.hash()` work as expected in all supported
browsers.