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)
I'm finding this text kind of confusing. As I understand the API, what the application
actually does here is to frob the ICE settings and then the browser does gathering
for all the candidates but doesn't surface the others, right?