feat(*): ability to bind once an expressions

Expressions that start with `::` will be binded once. The rule
that binding follows is that the binding will take the first
not-undefined value at the end of a $digest cycle.

Watchers from $watch, $watchCollection and $watchGroup will
automatically stop watching when the expression(s) are bind-once
and fulfill.

Watchers from text and attributes interpolations will
automatically stop watching when the expressions are fulfill.

All directives that use $parse for expressions will automatically
work with bind-once expressions. E.g.

<div ng-bind="::foo"></div>
<li ng-repeat="item in ::items">{{::item.name}};</li>