add dew point from old library

[ladyada]

handy code, lets recycle! https://github.com/adafruit/Adafruit_Python_BME280/pull/20

@kattni this is a good one for ya, you can test on a microcontroller (no need to use pi even tho original library was for pi), do it for celsius only, skip the _f version

[kattni]

Added and tested! I'll update the example to include it as well.

[kattni]

Done!

[kattni]

Build and PyPi deployment successful.