Warn if event deletion fails
More robust event comparison

Event occurrence might be replaced with a new one including
same start and end time plus uid, but different recurrence id
and some other information.
Unit test not to try removing already deleted events

Also remove full event series, not leaving event instances with recurrence
ids behind.
[nemo-qml-plugin-calendar] Fix event loading on days with occurrence and exception

Loading events from a day failed if it had multiple instances of single
uid, with different or same recurrence id.
Merge pull request #48 from pvuorela/master

Fix loading events from a day if there are multiple instances with same uid + small changes