I'm trying to consolidate the various schemas now, but I don't know how hard that's gonna be, so I want to do it on a separate branch and in the meantime get this pull request merged.

My plan (on another branch) is something like:

`ckan/logic/schema.py`:

    _base_package_schema()
    default_create_package_schema()
    default_update_package_schema()
    default_show_package_schema()

Then in `IDatasetForm` and `DefaultDatasetForm`:

    create_package_schema()
    update_package_schema()
    show_package_schema()

`DefaultDatasetForm`'s implementations of these three will return `None`. In `package_create()`, `package_update()` and `package_show()` if the plugin doesn't have the `*_package_schema()` method or if it returns none, then fall back on the `default_*_package_schema()` method from `schema.py`.