Add a nice wrapper for zoomus

[Mr0grog]

I might be getting overcomplicated here. In #73, I added a bunch of error handling code for Zoom, but it felt kind of awkward. I reworked that here into a FancyZoom class. You use it just like zoomus.ZoomClient, except the API calls return parsed data on success and raise exceptions on error.

No need to explicitly parse:

# Previously:
zoom.user.get(id='abc123').json()['id']

# Now:
zoom.user.get(id='abc123')['id']

No need to check and handle error statuses with lots of extra conditions:

# Previously:
response = zoom.user.get(id='bad_id')
if response.status_code >= 400:
    # Do something indicating the error

# Now:
zoom.user.get(id='bad_id')  # raises `ZoomError` with a nice message

This also moves file downloading into the new class, since it makes more sense there.