Create calendar analyzer for availabilities

https://trello.com/c/FulwCRAw/26-refactor-availability-and-pickup-methods-with-new-object
Same caching problem here. Just save off the number, and then use rails fancy to access dynamically:

``` ruby
ONE_WEEK = 7.days
ONE_YEAR = 365.days
# then use as
ONE_YEAR.from_now
```