Fix servo compatibility

### Compatibility with Servo.h

Because Servo Library uses the same callbacks of DueTimer, we provides a custom solution for working with both of them. However, Timers 0,2,3,4 and 5 will not Work anymore.

You will need uncommend the line in `DueTimer.h` in `DueTimer` folder inside the `Libraries` folder. Uncomment the following line in `DueTimer.h`:

```
#define USING_SERVO_LIB	true
```

Fixes #10