Moved the JSON constants to a new interface

There are needed on replayer side to unparse the JSON. So far,
they are hard wired, which is bad. With this, we can import these
constants and not need to import the whole ClientRecorder class
just for them.

These might be moved to a special "util" or "common" project if
there will ever be a need.
Removed unused import
Moved the EventType to it's own file
Renamed EventType to Events
Moved the change origin to their own interface
Shortened the name of the change origins
Merge pull request #66 from caiusb/json-constants

Moved the JSON constants to a new interface