Adds generic fetch_vertical method and re-organizes code structure

[aborem]

Closes #48

fetch_vertical

• Takes in a vertical, request_params for to be sent in the actual HTTP request, and params, which are optional key-word arguments specific to each method.
• Works by using the vertical's plural name to generate the method name and calling that with the latter two arguments described above. May seem a little risky to call a method based on a name, but given that we check if the vertical is supported and generate the method names based on the vertical names, I think it should be okay (and I wrote tests). As always, open to feedback!
• For the sake of consistency, I added a request_params argument to every fetch_ method (which I think is smart anyway since our pardners may want fine-grained configurability and may have familiarity with the APIs).

Exception reorganization

• Moved all exception definitions from services/base.py to exceptions.py
• Made the list arguments in the exceptions constructors optionally lists to get around having to generate one element lists to raise the exception.

Misc

• Made helper method (is_vertical_supported(...)) for checking if a vertical is supported using _supported_verticals since it's not unexpected that our pardners may want to check before adding a vertical
• Renamed fetch_athlete_activities(...) to fetch_physical_activities(...) for consistency
• Moved request_params to be first argument in all the fetch_ methods.