Feature/associations

Changes in associations:
* JSON support.
* Now an empty or non-existing attribute list in a *association discover context request* will behave just like a normal *discover context request*.
I'm not sure this is what we wanted.
If I understand this correctly, if associations are used and there are no attributes in the incoming request, a normal discovery is performed.
That is not what we want. We want an 'association discovery', but, all attributes must be returned for the discovered entities.
The problem before was that nothing was found in an 'association discovery' without attributes.
@fgalan, please help to clarify this.