Sensors: add parameters to calibrate the z-accel

This is redundant with the 3 parameters of the accel bias,
but by applying it as a separate term it avoids repeating
the six-point calibration on a regular basis.

This is useful especially for VTOL aircrafts where having a
good z calibration is especially critical for a good autonomous
hover.
Calibration: calculate the z-accel offset when leveling

This calculates the offset that will make the z-accel average
gravity after leveling.
Calibration: connect the bias leveling method to a button

The user can either click a button to calculate the board rotation with
or without zeroing the z-accelerometer.
Attitude: move the zero gyros option check box

It was grouped with the leveling button (historically) but is
an independent option so doesn't really belong there.
SetupWizard: use bias and leveling for MR

This method is better for multirotors as it results
in a better hover.
Merge pull request #1001 from peabody124/level_calibration

Level calibration