add callback to Hoist.log

fix for half of issue hoist/Hoist#38
add callback and test to events.raise

fixes the other half of Hoist/hoist#38
add documentation for callbacks
fix link
Merge pull request #3 from hoist/feature/log_callback

Add missing callbacks
Merge branch 'master' of github.com:hoist/hoist-connect into feature/connector_loading_by_key