Fix getInitialState in connect mixin

For now, initial state in component is equal to `getInitial` method's result in store. It was fixed. For now, if second argument in `Exim.connect` is key(string), then we set only this key from store as initial value, if array of keys - each key of array.

**Store**
``` javascript
getInitial: function ()  {
  return {selected: []}
}
```

**Component**
``` javascript
mixins: [Exim.connect(store, 'selected')],
componentWillMount: function () { console.log(this.state); }  //{selected: []}
```