Add tests and readme to card client

Basing on !1 (merged) add tests and documentation to card client to allow merge to master.

This documentation should enable a non-DevOps technical user to make user of the script without guidance (IT managers of colleges / users wanting to use the Card API for system-to-system integration).

The script is fairly simple - querying Lookup for a given institution or group and then using the Card API to make batched requests by CRSID - so it should be possible to achieve high levels of test coverage.