Pull up/down working

Thanks to feedback from https://github.com/akarnaukh if found the error in the pull function. I checked with my board, pull is now working nice without pull-up resistor. Also updated README.md
For my side I am now fine with this library - I will use it. Let me know when you pushed it ruby-gems.
Let me know your process with ruby and cubietruck :-)
:+1: