I2C Fastmode on lm4f Stellaris

I managed to implement FastMode in lm4f devices overloading the function:
void setModule(unsigned long);
void setModule(unsigned long,bool);

This should FIX #282 and be totally backward compatible.

Moreover there was a big delay in the red/write function. Using delayMicroseconds and the define SLOWMODE_DELAYUS is now set to the lowest always working time (From 1ms to 0.2ms).

All the performance data and some code explanation can be found in the link on top of my README.md file.

I only have a lm4f120h5qr and the sensors on a GY-80 so it may require to be tested on the other lm4f board.