New functionality:
* Credentials persisted to disk by default and loaded automatically when authenticating with `client.authenticate()`
* Get a list of guardians (for a given site or for your accessible sites)
Example: `client.guardians(sites=['derc'])`
* Get a list of audio files (for a given guardian and time period)
Example: `client.guardianAudio('f49300264d7d', start='2020-01-01T00:00:00.000Z', limit=5, descending=False)`
Bug fixes:
* Tags endpoint returns correct labels
Documentation: https://rfcx.github.io/rfcx-sdk-python/