[Bug] Add a Rescan method to Manager interface : Fix Issue #2 

- Add a new field (browsers) in struct dlr_upnp_t_ to keep track of
 the gupnp_control_points created for each network interface since
 there is no other way to retrieve them.
- Documentation updated.

Signed-off-by: Sébastien Bianti <sebastien.bianti@linux.intel.com>