Probing 

Finally I got this in to the dev branch.
Support for probing g codes G38.2, G38.3, G38.4 and G38.5 (see for more details http://linuxcnc.org/docs/html/gcode/gcode.html#sec:G38-probe).

Connect your probe to Analog pin 5 and gnd, since analog pin 5 is reserved you need to comment out the line
```
//   #define PROBE_38  // Probing disabled by default. Uncomment to enable.
```
in pin_map.h and recompile GRBL to make it work.

The probing result are written to the serial console in the following format
```
<Probe,MPos:5.3063,26.9594,-0.7438,WPos:5.3063,26.9594,-0.0313>
```

The idea behind this is to use it with my GRBL Levelling project (https://github.com/henols/GrblAutoLeveller) to make a crocked table strait, so i can use my CNC for milling pcbs and engravings.