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)
Why does the generality bother you?  Having just two levels of policy is fine here.  I think that's all we want.  And what you described is exactly what the text says, assuming that we define a policy that hides host and server reflexive candidates as the text stipulates.