Add some useful comments, and reformatted some code

[christophergeiger3]

Moved the location and name variable definitions to the top of the file. Annotated the add_item, remove_item, list_item functions. Dropped an else statement (originally line 33, in the class ToDo, after if self.exp_date is None) which is never invoked.

[aesophor]

Thanks for cleaning up my code!! If there's any way I could further improve it, please let me know :tada: