1st TX test: send '0'/'1' every 90ms and blink.
WIP: Start from Arduino example.
WIP: adapt to nrf51.
WIP: update makefile.
WIP: remove a few compilation errors (types...).
WIP: compilation OK, to test.
WIP: add IMU access in main - not working yet.
WIP: cleanup.
Make types more coherent (use uint8_t VS byte).
WIP: access to regs but not to sensor yet.
WIP: IMU OK, print x accel data.
Cleanup I2Cdev.
Getting close to a cleaner version.
Print compass heading.