Add new method bindObj that uses the $parse service for binding deep scope objects.

@agrublev: thanks for your great code!

The current `bind` method can bind only values at the top level of the scope.
The new method `bindObj($scope, key, obj, def)` can also bind values in sub-objects of the scope. Third parameter `obj` is an Angular expression for the value to be bound.

Example:

    $store.bindObj($scope,'viewType','model.view[7].type','cardView');

This will bind the value at `$scope.model.view[7].type` to the local storage key `'viewType'`.

Inspired by [this](https://github.com/firebase/angularFire/pull/18) and [this](https://github.com/firebase/angularFire/pull/29).