Make it provider agnostic

[awlx]

To add #27 we should make the project provider agnostic and the config be able to select one or multiple providers with different credentials.

[rucarrol-goog]

Task list to track conversion of existing code in unifi_respondd/unifi_client.py to be Agnostic.

• Design provider agnostic API #30
• Sketch out API via Abstract Base Class #31
• Rewrite unifi_client.py to inherit ABCs new API #32
• Add unit tests for unifi_respondd/unifi_client.py #33