Expose store ACTION_LOG_LENGTH as a user-settable param, and add a :clearLog method to the store

[sayhisam1]

At times, it is useful to know what actions were successfully applied to a store (e.g. to fire highly specific listeners that correspond to specific actions). At present, however, the action log is very limited and of fixed length.
It would be useful if there was a way to change the log length, and also clear the log if needed.