Delete user associated data upon deleting user

When user is deleted, user map is deleted.
When map is deleted, all associated location pins are deleted.
When location pin is deleted, all associated blog posts and photos are deleted.
When user is deleted, all trips are deleted.
When trip is deleted, all location pins are deleted.
Merge pull request #57 from femmestem/feature/dependent_destroy

Delete user associated data upon deleting user