Add MPU9150 folder: MP6050 with magnetometer. 

Hi @jrowberg & @udawat!
I followed your advice and made a separate folder from MP6050.
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.
Let me know if I can improve this pull request anyhow ;)
Cedric.