Add Sprout-specific checks for package properties

[martonvago]

When checking package properties with checks.check_package_properties in Sprout, add the following Sprout-specific checks:

• extra required fields: name, id, title, description, version, created (+ resources already required)
• required fields must not be empty / blank (e.g. the empty string is not a valid value for a required string field)

These should not modify / know about the JSON schema.
The checks should return a CheckError with appropriate attributes.