[#627] Allow to create private datasets

The private field was not present on the default package schema, used by
default_create_package_schema, so if you defined private in your package
dict it was ignored. This commit adds it to the default package schema
(ignoring it if not present).
[#627] Fix test broken due to default schema change
PEP8
Merge pull request #627 from okfn/627-create-private-datasets

You can not create a dataset private by default