[libsocialcache] Add e-tag support to CalDavCalendarDatabase

Add eTags() and insertETags() to the public API.

Also add removeIncidenceChangeEntriesOnly(), which will not remove
etags from the database, unlike removeEntries().
[libsocialcache] Bump lib version
Merge pull request #49 from nemomobile/caldav-etags

[libsocialcache] Add e-tag support to CalDavCalendarDatabase