Made compilation on OSX a little easier and updated to use RbConfig

OSX Compilation was not supported out of the box so I just added the correct paths and a small section to find the paths based on the OS and architecture running on the host machine. Running on OSX also requires a few paths to be added to ~/.bash_profile so I updated the readme to describe these.

Config was removed from Ruby some time ago so all those calls throw warnings, updated them to use RbConfig instead.