Added CollabSession.start() and stop().
Added CollabSession.start() and stop().

substance/substance,andene/substance,andene/substance,michael/substance-1,jacwright/substance,abhaga/substance,abhaga/substance,stencila/substance,podviaznikov/substance,substance/substance,podviaznikov/substance,TypesetIO/substance,michael/substance-1,jacwright/substance,TypesetIO/substance