Fix simple export for session w/o proposals, add start and end time.
Fix simple export for session w/o proposals, add start and end time.

pysv/djep,pysv/djep,EuroPython/djep,EuroPython/djep,EuroPython/djep,EuroPython/djep,pysv/djep,pysv/djep,pysv/djep