Caldav pgsql

Adds initial postgresql back-end support, but this is just an initial PoC that breaks mysql support for caldav in order to get postgresql working; related to issue #20. The existing mysql db has a column name `user`, which is a reserved keyword in postgresql, so renamed that to `username`.

TODO:
 - add mysql db update file to update `user` column to `username`
 - work out db-agnostic code for timestamps in the `last_change` column in `caldav_props`

See merge request !5