chore($q): rename `promise.always` to `promise.finally`

BREAKING CHANGE: the `always` method has been renamed to `finally`.

The reason for this change is to align `$q` with the Q promises library,
despite the fact that this makes it a bit more difficult to
use with non-ES5 browsers, like IE8.

`finally` also goes well together with `catch` api that was added to
$q recently and is part of the DOM promises standard.

To migrate the code follow the example below:

Before:

$http.get('/foo').always(doSomething);

After:

$http.get('/foo').finally(doSomething);

or for IE8 compatible code:

$http.get('/foo')['finally'](doSomething);