Merge pull request #368 from kh4/forPat

AQ32: fix wirish_time on newer compiler (caused attitude spin)
Merge pull request #370 from kh4/forPat

MPU60x0: increase calibration threshold as the old value is really tight...
Merge pull request #371 from AeroQuad/development

Preparing for version 3.1 release
Update README.markdown
Libmaple: cleaner fix for micros() race