add test for write_cheque

WIP

This commit adds test for writeCheque and deposit cheque. WriteCheque generates cheque but deposit_cheque returns empty string
Please install `flake8`, the python linter, it will catch things like long lines and non-standard spacing.  I'll add a config file to the repo (mostly because 80char lines drive me crazy, I'd prefer at least 100 if that's ok with you)