Address ICE concerns from reviews. Fixes #44, #46, #47, #61, and some of #52.

Tony: Explain, give example for ICE candidate pool (#46)
Magnus: ICE policy should be overridable by local policy (#52)
Explain ICE gathering phase (#44)
Replace updateIce with setConfiguration
Define behavior of setConfiguration, explain how changing ICE policy works (#47)
Default candidate pool size to zero (#61)
FWIW, I think there is also a third mode that might be desired, namely "no local", to prevent gathering of internal network details. Any intra-NAT calls would have to be hairpinned through the srflx address.
If we want to nail things down a bit more, I could enumerate exact the policies we want to support (all, relay, nohost) in this section.