Enhance process data logging

[garyfeng]

Given what EthanPad server stores (issue #2 ), we need to add the following to the messages:

• triggering events: these are the primary focus in our original keystroke logging design; EP, rather, focuses on the text transformations.
• text diff: the changeset seems to suffice, but we need to know more about it. See https://github.com/garyfeng/etherpad-lite/blob/develop/doc/api/http_api.md#creatediffhtmlpadid-startrev-endrev
• other GUI and system events: we need to log custom events via the same channel.