More configuration handling updates.

Configuration checking is now split into check, check_accounts and
check_property. All the default parameters are checked in the root
Configuration object.

More exceptions are catched during parsing. Useless check of
sqlite.path is removed (since a default value is provided if missing).
Modified the syncer for the updated configuration.

For now, it only considers the first defined account.
Added a parameter to bypass the accounts checking.

This is specially useful for tools working only on the database (like
pc_query) which thus never ask for a webdav password.