Python-cash-register

**Bugfixes**
- fixed 6: expecting `{price}` key instead of `{amount}`
- fixed 5: when the commands list is flushed they are removed from the `_commands` list

**Major change**
- Initial commit: project bootstrap

**Added features**:
- Implemented the base `Command` class
- Implemented the base `CashRegister` class
- Add initial support for the `Sarema X1` cash register model