Address ICE concerns from reviews

Tony: Explain, give example for ICE candidate pool
Magnus: Explain, give example for ICE policy
Magnus: ICE policy should be overridable by local policy
Explain ICE gathering phase
Replace updateIce with setConfiguration
Define behavior of setConfiguration
Default candidate pool size to zero
Fix build problem
Code review changes

ICE restart now happens automatically when needed
Explain how candidate policy works more clearly.
MUST have default candidate pool size of zero.
Fix minor issues identified in read-through

SDP generation code now generates new ufrag/pwd on config change
Spell out ICE candidate policies

Fix nits with Travis build failure and comments on previous versions of
this spec
Merge pull request #63 from rtcweb-wg/juberti3

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