Adding test CreateAttributeServiceTest.invalidConfigurationForIntegerAttributeType()
Adding tests InstanceServiceTest.classWithoutAttributes() and InstanceServiceTest.classWithoutAttributesUnknownAttributes()
Correcting some exception messages
Validation of test 'classWithoutAttributes' of Instance
Validation of test 'classWithoutAttributesUnknownAttributes' of Instance
Merge pull request #42 from mateuscomp/master

Validation of test cases: 'classWithoutAttributes' and 'classWithoutAttributesUnknownAttributes' of Instance.