Correcting some exception messages
Validation of test 'classWithoutAttributes' of Instance
Validation of test 'classWithoutAttributesUnknownAttributes' of Instance
Test cases: 'unknownClass', 'nullClass' and 'classWithKnownAttributesAndWithoutConfiguration' for Instance. Adding the validation AttributeValue in InstanceHelper
Merge pull request #43 from mateuscomp/tests

I will accept this pull request, but there are a lot of comments I have made.
Please, read them.