O295and o296 sponsorship start date validation

https://osraav.atlassian.net/browse/OSRA-295
https://osraav.atlassian.net/browse/OSRA-296

Model validation for Sponsorship.start_date 
- date can't be blank
- date can't be an arbitrary string
- date can't be later than the first of next month

plus rspec  and cucumber tests

Needs ruby 2.1.5
Me and Andrey paired on all this.
I preferred to stub today. So if I test for  five_days_ahead_this_month it will be this month.
Ex. if today is 30.11.2014   then I don't have five_days_ahead_this_month = today + 5.days. It will fail because it's next month.

But looking again, I could test future dates that not pass 1st of next month just by testing tomorrow. Even if it's 30.11.2014  or 1.12.2014, tomorrow day will still pas validations.