Change `target` ownership to strong, fix -test_no_exception_if_target_goes_away

I'd still [cast a vote][c] that this will limit the functionality of
SSArrayDataSource's KVOing abilities, but I think it's a worthy
trade-off to prevent a crash.

[c]: https://github.com/splinesoft/SSDataSources/pull/29#issuecomment-39678236
Delegate array datasource items ownership to internal object

Takes the [design][d] suggested by @erikprice.

[d]: https://github.com/splinesoft/SSDataSources/pull/29#issuecomment-39655048