Added two new arguments to allow print output from Games class to go to either stdout or to a log file.

##### The two new arguments are:

1.  --log-to-file: A boolean that when true diverts the print statements in the Games class to write to a log file.  When false the print statements write to stdout, as before;

2. --log-filename: A string that specifies the name of the file the print statements will write to.  Ignored if --log-to-file is false.

##### Reasons for change:

1. To keep a permanent record of the results of a Hunger Games tournament;

2. Increase in speed.  Writing to stdout a tournament of 700-odd rounds with the default players takes around 10 seconds.  Writing to a log file the same tournament takes under a second to complete (NB: The commit comment about two orders of magnitude was comparing writing the text file to running app.py from IDLE, which is much slower).