Add an abstract base class to make deleting objects easier.

Similar to AdminObjectEdit, but not quite as polished for now.
Build the deleted message before deleting the objects, so they can be used to build a better message.

This does not bother passing around the objects as the message building methods can always call getObjects().
Clean up the $objects property comments.
More consistent method names for building the deleted message.
Add methods to allow easily building delete confirmation messages without the use of AdminDependency.

These messages look similar to AdminDependency messages, but should be more flexible, and are completely optional.
Merge pull request #24 from isaacgrant/admin-object-delete

Add an abstract base class to make deleting objects easier.