Rework trickle ICE and m=/c= stuff

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Add canTrickle property
Add text regarding a=end-of-candidates
Clarify dummy/null address/port stuff
Merge branch 'master' into pr/42
Clean up some stuff after a second look
Text suggestion from Eric
Merge pull request #83 from rtcweb-wg/pr/42

Rework trickle ICE and m=/c= stuff. Fixes #11, #18, #41, #80