Let crawler save OFT case list

* Oh hey, it just works with Struct, we don't even have
  an OFT::Case yet.
Add an OFT::Case that can add a summary
Make YearCaseList use the new OFT::Case

* Also fix a bug with a missing case on the by date page for competition 2011
Make CaseStore aware of OFT::Case

* Take the opportunity to pull some serialization concerns into
  a base class