Replace Gson by Jackson

[abourdon]

The Gson library is used to map objects to JSON representation. But this library lacks a key feature: let user to use getter/setter to get attribute value. Actually, only fields are used during mapping.

An issue has been opened from the Gson Github: https://github.com/google/gson/issues/232

I propose to use an other library, the powerful Jackson library, where this feature is already developed. As the Gson one, the Jackson library is already integrated to the Spring framework for latter refactoring if any.

Let me know

[abourdon]

Another argument, Gson cannot handle anonymous classes: http://stackoverflow.com/questions/39538677/why-does-gson-tojson-always-return-null

[abourdon]

Library is approved, we just have to have additional time to do it ;-)