Add bob-walker.py

Through an amazing oversight, we have not yet created a bob-walker in Python. This Pull Request attempts to address this egregious error using something that one hopes will become the reference implementation of a bob walker.

This implements all the modes available in the Perl version, with three notable new features:

- If bob is being drunk and random, the amount of slurring varies. This
  makes it possible for bob to be drunk AND consistent which should
  be an allowable state.
- bob will only say one million words before he gives up. If you'd
  like him to stop earlier, then just press CTRL-C at which point
  he's going to suggest that it might be PUB TIME!
- If bob encounters an error, we don't care. We're just going to
  stop what we are doing and suggest having another pint. This
  probably won't stop the error occurring, but we can always fix
  that later.
On reflection, I've split that out into #15. It shouldn't be in this PR.