Datamapper DataMapper

Allow to properly overwrite Model.auto_migrate!

Previously, calling DataMapper.auto_migrate! did
not call .auto_migrate! on all models descendants.
Instead, it called

auto_migrate_down!(repository_name) auto_migrate_up!(repository_name)

explicitly. With this change, these 2 methods will
still be called by .auto_migrate! but because the
auto_migrate! method is now used to invoke the other
2, it's now possible to truly overwrite it.

[#1225 state:resolved] http://github.com/datamapper/dm-core/commit/547627dde7bbd731eacb6c2...