Added function to calculate real/absolute pressure at sea level

I added a function that gives the real pressure at sea level.
Reason: The BMP085 is configured for usage on sea level, so we have to do a compensation calculation to consider the altitude of the actual sensor position.

For more information take a look at the BMP085 datasheet chapter 3.7: "Calculating pressure at sea level" http://www.adafruit.com/datasheets/BMP085_DataSheet_Rev.1.0_01July2008.pdf