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.
 Fixing some problems on merge #43
Merge pull request #44 from mateuscomp/tests

 Fixing some problems on merge #43
Removing the name 'default' of exception messages of instance tests
Removing the name 'default' of exception messages of instance tests
creation of test instanceWithValidValuesForTheConfigurationOfAttributesText
Merge between the previous versions
Creation of test instanceWithValidValuesForTheConfigurationOfAttributesLongText
fixing the assertion default name for instance tests
Implementation of ServiceFactory
Fixing the name of the methods for obtaining instances in ServiceFactory
Test cases: 'unknownClass', 'nullClass' and 'classWithKnownAttributesAndWithoutConfiguration' for Instance. Adding the validation AttributeValue in InstanceHelper
fixing the assertion default name for instance tests
Assigning a default value if available, an AttributeValue case is null
Fixing the refactorings needed for merge # 50
fixing the assertion default name for instance tests
Implementation of tests involving instances and configuration of attributes
Fixing the refactorings needed for merge #52
Implementation of ServiceFactory