feat($parse): Support getter/setter functions in parse expression.

Adds support for a syntax to define a property accessor in expressions, i.e.
a getter/setter pair. The straw man syntax is @foo supporting getFoo and
setFoo. Property accessors can be used in ng-model bindings, e.g.
  <input ng-model="obj.@foo">
Binding to obj.getFoo() and obj.setFoo(value) respectively.

Closes #768