Replaced Timestamp with Due, removed Priority and other minor changes

Rewrote list() to display due tasks and other tasks separately.
Implemented done().
Changed help() to display actual help (minimal, but still help).
Added print_config().
Moved PydoController logic to Pydo itself.
Changed default behaviour to display help instead of list.
Removed all traces of timestamp and priority.
Added parse_due() to Parser.
Replaced use of PydoController with Pydo.

Updated readme.md accordingly.