[rename] for FEMAttributeMapping internals
[add] initial spec for FEMAttributeMapping
[add] shortcuts tests for AttributeMapping
[fix] AttributeMapping specs type
[add] spec for URL mapping NSNull check
[add] spec for date mapping NSNull check
resolve #18, resolve #19
Merge pull request #20 from Yalantis/feature/invalid_mapping

feature/invalid_mapping