ComplementaryFilter: use z-axis correction from accels

This doesn't matter normally when board is horizontal but
when at strong pitches or rolls this correctlys the
perpendicular axis.

Not applying to the integral component since during most flying
this won't be useful or could only add noise and the mag is
more useful for this axis.
Merge pull request #903 from peabody124/cf_pitch

ComplementaryFilter: use z-axis correction from accels