O294 sponsorship end date validation

https://osraav.atlassian.net/browse/OSRA-294
This is connected to:  https://osraav.atlassian.net/browse/OSRA-298

Sponsorship model validatation for end_date
- can't be before start date
- can't be blank

plus rspec and cucumber tests

Andrey and me worked on this.
Craig's code looks better to me.

My approach to this: every model method that changes the database should return the result or false/nil. I was trying to simulate the 'save' method behavior.