Added condition to check attributes setted to nil on attribute getter.

This change make possible to set an attribute to nil and make it return
nil even if a default value exists for that attribute.
Added possibility to pass objects when working with Identity Associations
Removing duplicated build

Its not necessary to include ruby 2.1.0 on versions to build because it
is already defined to build in matrix
Fixed Coveralls build
Merge pull request #61 from zertico/nil_value

Added condition to check attributes setted to nil on attribute getter.
Merge pull request #63 from zertico/coveralls

Coveralls
Merge pull request #62 from zertico/improving_associations

Added possibility to pass objects when working with Identity Associations