Implement store getter.

`Exim.createStore(...)` will return getter object. Setters will be available only in the store methods. If we create new setter methods in store, we can make them private, just adding them in **privateMethods** array. 

*Store:*
```` javascript
Exim.createStore({
  privateMethods: ['add', 'remove'],
  add: function (item) {
    this.set('list', this.get('list').concat(item));
  }
  /*...*/
})
````