Todos

@mkwiatkowski 
Fixes #10 
This is a bit misleading. First of all `days_left` method doesn't return number of days left for a given todo object. It returns a time object that is exactly one week before the task was created. If you feed that into `time_ago_in_words` this will show "7 days" for tasks created today. For tasks created yesterday it will show... "8 days" (creation date - 7 days). And so on. So this is not only confusing, but also incorrect.

Focus on the `days_left` method first. How to write code that will return 7 for new tasks, 6 for tasks created yesterday, 5 for tasks created two days ago, etc.? Don't think about the implementation of it just yet. TDD it: write a model tests for it first.

One you get that working [distance_of_time_in_words](http://apidock.com/rails/v4.0.2/ActionView/Helpers/DateHelper/distance_of_time_in_words) may be better suiting than `time_ago_in_words` here.