`Relation#to_key_values` transforms cells back into `KeyValue`s.

I don't expect this to happen much in production, but it is handy for
tests (and pulling this out to fixture support ends up dragging out
lots of guts.)
Merge pull request #93 from seomoz/phs/key-value-factory-methods-on-relation

`Relation#to_key_values` transforms cells back into `KeyValue`s.