Add todo item

[biblicalph]

Is your feature or refactor request related to a problem? Please describe.
A user should be able to add a new todo item.
If the user has connected his/her Google Calendar, an error should be returned if an entry exists on the calendar for the specified period. If no entry exists, a new item should be added to the user's calendar.

A to-do item has the following characteristics:

1. Title (required)
2. Description (optional)
3. Due date and time (optional). If present should be added to the user's calendar if authorized