Calculation spec

[thegcat]

Add a spec for the somewhat convoluted calculations of open spots.

Also see #77

[thegcat]

I've never worked with draper an I'm a little unsure what methods should be public or not in a decorator, feel free to adapt as needed.

[pietia]

Thanks. I will take care of Travis later on today. BTW, There might be some specs failing due to this #80 issue (basically - adding validations in Authentication leads to circular dependency which makes this https://github.com/eurucamp/eurucamp-activities-2013/blob/master/app/controllers/authentications_controller.rb#L24 code fail (btw2, that's the code 'borrowed' from our c4p app that we got from wroc_love.rb organizers) )

[thegcat]

I've updated the commit and made the method public and I've given it a better name.