Object editor nestedForm class: use Backbone.Form or this.schema.nestedFormClass

The goal of this pull request is to fix a bug I experienced using a custom Form class. My form used an `Object` editor, and this bunch of code (in this [file](https://github.com/powmedia/backbone-forms/blob/master/src/editors/object.js#L29)) caused the app to crash :

    render: function(){
        var NestedForm = this.form.constructor;
        ...

As my form did some custom things in the initialisation, when instantiating the nested form, I add some very curious bugs. This PR fixes this, and makes it possible to specify a custom nested form class (just in case). Documentation is updated.

PS: I builded the distributions, so there might be things that were not built before that are included in this PR, let me know if this is a problem.