consider addressbook_path setting

With this patch, the plugin considers the addressbook_path string in the account settings and enables synchronization with owncloud (from owndrive.com) for me. For now, this string cannot be set using the Jolla account manager but has to be set manually using a terminal. This might also be interesting for https://github.com/nemomobile/buteo-sync-plugin-carddav/issues/4 . However, only the first sync works so far, unfortunately.

If someone is interested to test the patch:
1. Install the patched package. If someone hasn't set up a build environment: [buteo-sync-plugin-carddav-0.0.6-1.armv7hl.rpm](http://kicherer.org/stuff/buteo-sync-plugin-carddav-0.0.6-1.armv7hl.rpm) (d1558ac54c1b7a0f7007c6b05fde3c49)
2. Setup the carddav account (ensure that the server setting has no trailing "/").
3. Get the ID of the new account using ```ag-tool list-accounts```
4. Set addressbook_path using ```ag-tool update-service $your_id onlinesync-carddav string:addressbook_path=remote.php/carddav/addressbooks/$your_user/$your_addressbook```
5. Sync your account