First basic JSON to Elfin Scala object serialisation implementation.
Implemented basic serialisation related unit tests.
Refers to trello card: ELFIN XML <=> Scala <=> JSON
Motivation for Elfin Scala object is solving the information loss when
converting ELFIN in XML format to JSON format and then back to JSON.

Know issues are: 

* XML attributes are transformed to XML elements. 
* Empty element with attributes only information disappear. 
* JSON arrays are not produced when the XML element list contains a
single element.