Update endpoints with better method signatures.

Certain endpoints require one or zero values to be passed in
the get request to the API. Instead of using a Hash data
structure, change the method signature to accept the single
required field or not require a parameter at all.

- Remove the params hash parameter in aggregate_sport method.
- Switch from hash to single argument in activity_data method.
- Update tests for aggregate_sport and activity_data endpoints.
- Use string interpolation to set activity id in URL parameter.
Refactor the checks for invalid parameters in endpoints.

- Add errors file to subclass ArgumentError as InvalidArgumentError.
- Include errors file in nikeplusgem require file.
- Update Client to look for valid access_token and app_id parameters.
- Include tests to validate errors occur with invalid constructor params.
- Change existing exception raises to use the new InvalidArgumentError.