fixed local video flickering before receiving stream

videoElement.onplaying event listener being used, sometimes a short lag
may be still seen, though it's been very occasional in my tests.
Don't display the signin button until we've finished loading the page.
local video element displayed for video calls only
Merge pull request #225 from n1k0/fix-local-video-flickering

fixed local video flickering before receiving stream
Fix typo
Merge pull request #228 from Standard8/signin_stable

Don't display the signin button until we've finished loading the page.