Check if NUM_ANALOG_INPUTS is equal to 8

If not they would need to update the Arduino IDE - see: https://github.com/arduino/Arduino/pull/2148
Use Wire.setClock to set I2C frequency when using Arduino 1.5.7 or above

See: https://github.com/arduino/Arduino/pull/1912
Updated some comments and added static to PWM_FREQUENCY and PWMVALUE
Move calculation of accAngle inside updateIMUValues() and check if data is ready when calibrating gyro
The motors are turning opposite, so the other current sense value must be subtracted