Adding calibration code

Hello Philipp,

I've read an article about the TEMPer device on the March issue of Linux Journal, which is based on your code.
I've given a try for it and it's working great.

However, as it is often the case with these cheap devices, my TEMPer needed some calibration to show a correct value. So I looked into your code and made some modification to include a simple linear calibration formula.

Basically TemperDevice class reads a configuration file /etc/temper.conf for calibration parameters (which are decided beforehand by other means), stores them into calib_data attribute. And when get_temperature() is called, it checks if there is a calibration data for this device ( had to add a new argument to this method to indicate what "this device" means), and if there is one, applies it to calibrate temp_c.

I'm new to Github and this is actually my first pull request, so there might be several points that should be done in better way. Any comments/suggestions or modifications will be appriciated.

Regards, 
Joji Monma (GM3D on Github)