Rotate frame

[jbcolme]

I've being playing with the code trying to rotate the frame. Added some functions to imu.cpp and imu.hpp to make the calls to the api for that. I did not add all the frame rotation calls, and I'm still testing it. But I thought you may find it usefull. This is my first pull request on a different branch. I hope that what this does is create a branch in your repository (if you accept of course) so you can just take a look. Cheers.