Addressed matching names of file and test classes

with the name of product class.
Improved the test names.
Renamed Key to KeyMeta.
Introduced the GetKeyMetadata method.
Implemented the GetKeyMetadata method of the Article class.
Implemented the GetKeyMetadata method of the Bookmark class.
Implemented the GetKeyMetadata method of the CompositeModel class.
Implemented the GetKeyMetadata method of the Keyword class.