Merge pull request #1 from artemyk/master

Dataframe extension integration
Merge pull request #2 from artemyk/master

Avro bug fixed
Updated package files.
Revamped logging. Refactored client.
Added `Client.append` method.
Added `AvroWriter` class.
Fixed and added Avro tests.
`Client.append` test fix.
`AvroWriter` can now be used as a context manager.
Split `KerberosClient` into extension. Added support for extra requires.
Documentation.
Test check fix.
Fix travis test skipping logic.
Idem.
Bah. Travis is hard.
Removed mocks from Kerberos extension.

These make it difficult to know when the extension is actually loaded.
Minor fixes