kelvin to eV

Hello all. 
This pull request adds a new equivalency to convert between kelvin and eV. In my view this conversion comes to play in many occasions and is pretty useful. 
If something needs to be added further, please let me know! 
Thank you! 
The problem is not the missing keyword (although the example might be clearer with it), but that it is using `t_K` while the line above defines `t_k` (capital vs lower case).