#27: just renamed StringNumbersTest to StringNumberTest for consistency
#27: refactored StringNumber and affected classes.
Introducing a generic value type in StringNumber allows to make StringInteger and StringDouble even shorter.
#27: worked through the tests
#27: fixed all the failing TokenizerTest tests
(besides testRemovesCitationsPreservingDescriptionTitleAuthor -> issue #32)
Closes #27.