Correct start sorting for non-expanded get_events()

@thet This follows up on our conversation and your hints. Fixes #114.

Please note that the plone.app.event.tests.test_portlet_calendar.RendererTest failure is already on master before this pull request ;-).

Commit de1512f was not completely correct in documenting the bug. Commit 814eb29 rectifies this and also avoids some race conditions in date sorting, in addition to actually fixing the bug.