Implement media handover between Wifi and Cellular Data for a live call

[atsakiridis]

Currently reachability features work outside a call. If inside a call while network changes, the call is terminated.

Maybe a better approach would be to re-invite the other party. But what if it was an incoming call? Are we guaranteed that the call would reach it's destination? I'm a bit concerned that trying to automate that might lead to bad user experience (i.e. all of a sudden a call will turn from established to ringing (in case of client-to-client), etc)

@deruelle, @gvagenas, @ivelin: thoughts?

[atsakiridis]

Call handover functionality is a big undertaking and should be implemented post 1.0.0 GA. I'm adding it to 2.0.0 GA for now