In line with ngSrc and ngHref, this new directive ensures that the
`srcset` HTML5 attribute does not include a pre-interpolated string.
Without it the browser will fetch from the URL with the literal text
`{{hash}}` until AngularJS replaces the expression inside `{{hash}}`.

Closes #2601

Extend ng-options with a new clause, "track by [trackByExpression]", which can be used when
working with objects.  The `trackByExpression` should uniquely identify select options objects.
This solves the problem of previously having to match ng-options objects by identity.
You can now write: `ng-options="obj as obj.name for obj in objects track by obj.id"`
The "track by" expression will be used when checking for equality of objects.

Examples:
<select
    ng-model="user.favMovieStub"
    ng-options="movie as movie.name for movie in movies track by movie.id">
</select>

scope: {
  user: { name: 'Test user', favMovieStub: { id: 1, name: 'Starwars' } }
  movies: [{ id: 1, name: 'Starwars', rating: 5, ... }, { id: 13, ... }]
}

The select input will match user favMovieStub to the first movie in the movies array, and show
"Star Wars" as the selected item.

Fix the check for overrides so it is able to handle the empty string

Closes #2575

BREAKING CHANGE: css classes foo-setup/foo-start become foo/foo-active

The CSS transition classes have changed suffixes. To migrate rename
.foo-setup {...} to .foo {...}
.foo-start {...} to .foo-active {...}

or for type: enter, leave, move, show, hide

.foo-type-setup {...} to .foo-type {...}
.foo-type-start {...} to .foo-type-active {...}

Fix a check inside render for select elements with ngOptions, which
compares the selected property of an element with it's desired state.
Ensure the placeholder, if available, is explicitly selected if the model
value can not be found in the option list.
Without these fixes it's up to the browser implementation to decide which
option to choose. In most browsers, this has the effect of displaying the
first item in the list. In IE9 however, this causes the select to display
nothing.

Closes #2150, #1826

Adding a $includeContentRequested event in order to better keep track of
how many includes are sent and be able to compare it with how many have
finished.

closes: #2304

Check if the object is array-like to iterate over it like it's done with arrays.

Closes #2546

Closes: #2492