Added function to calculate real/absolute pressure at sea level

[F481]

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

[F481]

Forgot to make the altitude parameter variable.
Best would be to use a global variable where the user could set the altitude.

[tdicola]

Thanks for sending the pull and apologies it couldn't be reviewed sooner. I've actually refactored the BMP085 library a lot so it supports both the Pi and Beaglebone Black. In the process I also added the read sealevel pressure function like in this pull. Take a look at the new library at https://github.com/adafruit/Adafruit_Python_BMP if you're curious. I'll close this pull since this older library is now deprecated. Thanks again for sending the pull and suggesting the new function.