Added a "Start with Add button" option

This option causes Archetype to show "+ Add" button instead of default fieldset.
The button is only shown if there are no fieldsets -- subsequent fieldsets are added using standard control bar.

There are two reasons for that:

1. I find undeletable default fieldset confusing.
2. It completely sidesteps issue imulus/Archetype#68.  
Proper fix for that issue is rather hard as some editors may change fields on their own when loaded.

The UI design is:
![Add button design](https://cloud.githubusercontent.com/assets/166396/2586655/fe30ecb6-ba06-11e3-9cd1-52febad2a383.png)