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! 
You shouldn't use scipy's `constants` here because scipy is an optional dependency of astropy, so it's not guaranteed.  Also, `astropy` already has a constants module with `k` defined, and it's better to use this consistently in astropy.

Instead, use ``math.exp(1)`` for `e`  and ``astropy.constants.k_B.value`` for `k`