added test for Statement creation of rated authors
some more tests for Statement
fixed test
replaced List of tags with a Set

A statements tags don't have an order and should not be allocated more than once. So a Set is a better choice. This also makes it easier to compare statements since the order of its tags doesn't matter any more.
made `tags` available as a `List` of `Tags` sorted by name
Merge pull request #15 from MarcHauptmann/statement_tests

Improvement for Statement.create and fix for issue #9