Added sound engine, startup melody, eSky features and improvements 

<b>NEW FEATURES:</b>
- Sound engine which allows to play a melody specified with a note array
- Boot sequence sound: adjustable as you want, standard is "do-do-di"
- Sensitivity of flying when using eSky transmitter (ET6I) is changeable with the CH5 knob (from dfff180)

<b>IMPROVEMENTS:</b>
- Added jodell's changes for more responsiveness and a X mode when using the eSky transmitter
- Further adjustments of min and max values

<b>FURTHER NOTES:</b>
You can disable the boot melody by removing the "#define ACTIVATE_STARTUP_MELODY".
The min and max values for the sensitivity are also adjustable. The sensitivity is interpolated from the min and max value.

<b>NOTES:</b>
(for Tobias)
I was not able to use the clock division, if I used a higher values as 1, it stopped working. The ratio is configurable within the motorsBeep function. The functionality of playing the start up melody can be switched on and off by a define in the config.h - file.
Link to old pull request from bitbucket: https://bitbucket.org/bitcraze/crazyflie-firmware/pull-request/6/added-sound-engine-and-startup-sound/diff