Added internal logging

Using internal logging instead of TES5Edit's logging system.  This is a
superior choice because it lets me save the log each time the script is
executed and doesn't slow down script execution.  Some messages are
still printed to TES5Edit's log for clarity in certain situations.

Logs are saved in Edit Scripts/mp/logs/merge_<date>_<time>.txt.  Logs
can be viewed during script execution by clicking on the Show Details
button.