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)