Added barometer calibration functions and example code

I added barometer calibration functions to obtain more precise altitude measurements (and barometer readouts). Please see the example source \examples\\barometer_calibration\\barometer_calibration.ino   

The calibration has been obtained by other people before and I mentioned their work in my source comments.
I just thought to package everything in one place and merge it back within the original Sparkfun library.

The example source contains also an output sample (bottom of the source).

I've got better values from the sensor after the calibration (suggested in datasheet) .  if someone could check this with its own sensor I would be happy to have some feedback.
Ciao,
Mario