Add TeVCat

TeVCat ( http://tevcat.uchicago.edu/ ) is an online catalog of TeV gamma-ray sources that is continuously updated and I would like to add it to astroquery.

I already have code to make astropy tables from their JSON data.
At the moment I do `View -> Developer -> JavaScript Console` in the Chrome browser and type `jsonData`.
The one thing I don't know how to do is how to get the data from Python.
@keflavich Do you know how to do that? Wout `TeVCat` be acceptable for `astroquery` or do you think it's too niche?


The `json.loads` call still doesn't work on Python 3:
```
TypeError: the JSON object must be str, not 'bytes'
```
@keflavich Help!?