For Benji - Added module scoped API string.  Enhanced error checking of parameters p...

...assed to Client.instantiate/track/label/score.

@bstein-sift I added a module scoped api_key variable to mimic [Stripe's Ruby API](https://stripe.com/docs/api?lang=ruby#authentication) and for parity with the sift-python client library.

I removed the api_key.to_s as we don't know what users will send in that field and we do not do this with event/user_id parameters to other Client methods (also for parity with python lib).
nit: I think the `to_s` is unnecessary here