res_stasis.c: Add new type 'sdp_label' for bridge creation.

res_stasis.c: Add new type 'sdp_label' for bridge creation.

Add new type 'sdp_label' when creating a bridge using the ARI. This will
add labels to the SDP for each stream, the label is set to the
corresponding channel id.

Resolves: #91

UserNote: When creating a bridge using the ARI the 'type' argument now
accepts a new value 'sdp_label' which will configure the bridge to add
labels for each stream in the SDP with the corresponding channel id.