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! 
How about the following:
```
This equivalency allows conversion between temperature and its equivalent in energy (i.e.,
the temperature multiplied by the Boltzmann constant), usually expressed in electronvolts.
This is used frequently for observations at high-energy, be it for solar or X-ray astronomy.
```