Missing documentation and important constants on TangoServiceClientNodes

[adamantivm]

Trying to understand the expected behavior of TangoServiceClientNode and its callback is currently very difficult and not fully documented.

Two things that I am running into now, which I can only resolve by looking at the source code of TangoRosStreamer and copying how it is implemented there:

1- Understanding what is the expected behavior of callTangoConnectService
2- How to tell if the service started successfully or not
3- What is the meaning of the status codes returned by onTangoStatus

I suggest clarifying this in the wiki and also adding the following enum as part of the TangoServicClientNode:

// Symmetric implementation to tango_ros_node.h.
enum TangoStatus {
    UNKNOWN,
    SERVICE_NOT_CONNECTED,
    NO_FIRST_VALID_POSE,
    SERVICE_CONNECTED
}

[PerrineAguiar]

1 and 2 - Tutorial has been updated: http://wiki.ros.org/tango_ros_streamer/tango_ros_node#Start_streaming_Tango_data
3- Wiki has been updated: http://wiki.ros.org/tango_ros_streamer#Published_Topics

TODO: add the enum as part of the TangoServiceClientNode.