Adds issues_tests for testing GitHub issues.

Adds rake task for explicitly running issues_tests, which do not get run
via `rake test`.

Adds failing test for #46
Adds test for destroying a child record when the parent does not mixin
DestroyAt. No longer raising `undefined_method`.

Fixes #46

Updates CHNAGELOG
Merge pull request #48 from dockyard/46-undefined_method

Fixes deleting targets whose owner does not mixin DestroyedAt