Featurerequest: MIDRC calibration during startup

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My Spektrum DX5e seems to change MIDRC values from time to time (in range of 
+-50), but different for each axe. (It's not a problem of trimming - ok 
possible to trim against it, but it's kind of not perfect solution :))

I think it could be nice to have a setting to automatically calibrate MIDRC 
during startup. So it will read roll, yaw and pitch during initialisation and 
then store each value in an array and use that instead of MIDRC constant.

Original issue reported on code.google.com by [e-mail] on 27 Sep 2013 at 7:30