Add render on save only configuration

You can switch mode with shortcut `ctrl + alt + a`
Default mode is render on change

Add a checkbox in settings : 
![capture du 2014-05-22 01 30 24](https://cloud.githubusercontent.com/assets/2006548/3048054/f1e562c4-e13f-11e3-9216-855968097338.png)

Add information on the active mode in the bottom toolbar : 
![capture du 2014-05-22 01 31 16](https://cloud.githubusercontent.com/assets/2006548/3048061/13584fac-e140-11e3-870b-41d81d16c82f.png)