Topic/multiple objects

Hi,

This pull request implements multiple objects recognition with a single instance of a recognition/tracking nodes pair. It tries to share as much computation between objects as possible in order to maintain good performance. However, on lower end systems, the detector node can slow up the tracking node significantly.

It is also possible (through service calls) to enable/disable tracking for certain objects. However, right now it is not possible to:
- train multiple objects at the same time (that wouldn't be very useful anyway I think)
- add objects on the fly and/or give an initial set of objects to track, all objects that you wish to track should be present in the configuration file at the beginning and they will all be tracked by default. Both of these options may be interesting to add later on.
- single-shot mode only works with one object

Best regards,