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.

By now this push also contains the documenting testcase for #116. I've set up the test to pass for now.