Implement communication protocol between SensorNode and ManagementNode

### Why
Because there has to be one, obviously :-)
The ContextDomainGroup managed by a _ManagementNode_ needs to be modeled as a ThingDescription entity, _as well_.

### What
- [x] implement 3 default Thing description **properties**
- the GET on a base representation URI (`http:///coord/location/`) returns the description of the ContextDomain
- the GET on `/providers` returns the list of registered SensorNodes
- the GET on `/capabilities` returns the list of capabilities (ThingDescription modeled devices)
- [ ] implement 1 ThingDescription **action** - _join_ - by which SensorNode can register itself and its _capabilities_ (the devices it manages, modeled as ThingDescription entities)

### Notes
All communication payloads are transmitted as RDF triplets.