Adding Z Probe via sled mounted endstop

Hello. I would like to contribute this feature to the Marlin source code. It uses a sled-mounted endstop that rides on the X axis. There is code to allow the use of a latching mechanism, an electromagnet, to attach the sled to the X carriage during the Z probing process. More specifically, when the Z probing process starts, the X carriage runs out to the sled, engages the electromagnet, then proceeds with the probing. When done, the X carriage runs out to MAX X and undocks the sled. No more servos chattering endlessly or drooping down to clobber your prints!

You can read more about this feature on Thingiverse: http://www.thingiverse.com/thing:396692

If there are any issues with the code as I've written, I will be happy to incorporate changes prior to acceptance of these commits.

You can reach me directly via drcharlesbell@gmail.com

it need to retract probe after homing if ENABLE_AUTO_BED_LEVELING