DataReturn callback error on getData [0.2.7] (issue #46)

This version introduces multiple enhancements:

fixed error when try to prepare whole data on return
also fixed some export methods to handle data preparation

Details of additions/deletions below:
--------------------------------------------------------------

Modified:   src/Base/BlueObject.php
            -- Added --
                _prepareWholeData allow to use return preparation on all data in object
            -- Updated --
                _dataPreparation code refactor, fixed problem with null key and usage of _prepareWholeData method
                toXml use method getData instead of _DATA array
                toStdClass use method getData instead of _DATA array

Modified:   composer.json
            -- Modified --
                changed version from 0.2.6 to 0.2.7