Removes "properly selects columns" test, which was used in
@destroyed was being set when the object has a destroyed_at attribute.

Removes "only destroys and restores related dependences" test, as this
is covered by "destroys dependent relations with DestroyedAt", "does not
delete dependent relations with DestroyedAt", "restores relationships
with DestroyedAt", and "does not restore relationships with DestroyedAt"
tests.
Simplifies tests and test_helper by using blog fixture data.

Updates README to reflect the behavior of deleting a parent record with
depedent destroy, whose child does not include destroyed_at.

Removes test for restoring records without destroyed_at as they will
have already been deleted.
Removes tests for different relation types. These are encapsulated in
"destroys dependent relation with DestroyedAt" and "restores dependent
relation with DestroyedAt". It also adds test for destroying and
restoring a dependent relation with a through relation.
Adds note to README about destroying relations that do not include the
mixin.
Merge pull request #31 from dockyard/md-cleanup_tests

Cleans up tests