Implement whych with tests

 - Added routine to bscr/__init__.py
 - Document in 'bscr help_commands'
 - Tests for python_which, perl_which, bash_which
 - Create a command line entry point for whych
Account for .pyc files in py whych
python_which() now satisfies its test
Add replay to backscratcher

 - Document the routines in replay
 - Handle ^C better
Add trigger functionality to replay
Make trigger work in replay

 - Got it working
 - Skip unpredictable which tests
 - Document util.mtime()
Final tweaks