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 would change the description to something like 
` Sponsorship can not be created if date is fuzzy match`
or 
`Sponsorship can not be created if date is invalid because of an ambiguous or fuzzy match`
in order to make the intent of the test clearer