Wait for Z probe hit only the first time

Hi,

First, I'm using Repetier firmware for almost a year on my 3D printer and I find it really great !! So thanks for the hard work :-)

I've added a hall effect sensor to determine the Z height using this concept: http://reprap.org/wiki/CrashProbe.
You manually need to bring the probe/magnet down only the first time. Once done all others measures can take place without any user action.
For this purpose, I've modified the code to add Z_PROBE_WAIT_TEST_ONLY_FIRST_TIME in the configuration file and modified waitForZProbeStart.

In addition, I've added a release probe message. In the original code if the probe was already in contact, you just return and continue. But if the probe is already in contact it can mean that the probe was too low and therefore you can't probe the height or in my case the probe has not been released/brought down or went up inadvertidly.

This is a minor modification but really usefull. I hope you'll consider a merge.

Regards, Pascal