Add MPU9150 folder: MP6050 with magnetometer (but without anymore file MP6050 related)

Hi @jrowberg & @udawat!
This pull request implements the suggestions from the [#56](https://github.com/jrowberg/i2cdevlib/pull/56) (and [#57](https://github.com/jrowberg/i2cdevlib/pull/57)) request feedback.

The steps are on the [MPU9150 branch](https://github.com/honnet/i2cdevlib/commits/MPU9150) but I squashed them in one commit to make it cleaner on your repo if ever you merge it.

Have a good weekend !
Cedric ;)