Add 5 custom gcode command buttons to machine control panel

This adds 5 "custom gcode" buttons to the machine control tab. 
 * Saves to setting file
 * Multiple commands separated by ;
 
![custombuttons](https://cloud.githubusercontent.com/assets/456755/3962414/779ab8a4-276a-11e4-9fdc-4495212e3bad.png)