Implement persistable option in Mariontte FormView

By setting the persistsData property of a Marionette FormView to true, it will enable the storage and recovery of fields using sessionStorage, based on the className of the FormView. 

In order to persist a field add the persist: true attribute. 

```
var SignUpForm = Marionette.FormView.extend({
    initialize: function (...) {....}, 
    persistsData: true,
    className: 'get-started-form',
    data: { lastName: 'Legard' }.
    fields:  {
         firstName: {
             el: '#getting-started-firstname',
             persist: true
         },
         lastName: { 
            el: '#getting-started-lastname',
            persist: true
         } 
    } 
    ....
});
````
This will persist the FirstName field through page refresh until the browser window is closed, or explicitly removed by calling window.sessionStorage.removeItem('get-started-form') or by calling window.sessionStorage.clear()

Data explicitly passed in the this.data hash, will not be overridden. In the example, when the user refreshes, the last name will always render as 'Legard'