Version 0.3

• Leitner system implemented.
• Changed systems so that the system is with flags instead of names. In this
  way one can do random-leitner-inverse for example.
• All queries parameterized where possible, also changed deck databases to be
  consisting of ids and ascii only to let names contain every character.
• Updated readme with help.
• Removed the need for a config file.
• Better time representation in cli.py while listing decks.