Paging contact list

Fix for https://github.com/owncloud/contacts/issues/136 and https://github.com/owncloud/contacts/issues/334 that adds basic pagination for the contacts list.

When the contact list loads, it retrieves 15 contacts from each address book. Each time the new "load more" button is clicked, 15 more contacts will be retrieved from each address book, until there are no more contacts left to load.

While the paging limit defaults to 15 contacts per address book, it can be changed from the contacts settings.

Client side caching of contacts still operates, but cached contacts are now invalidated when the paging limit is changed as well as when the address book is modified.

While I see that pagination isn't under consideration because of other speed improvements to the contacts app, it was a feature that we required, and it seems that some others are interested in it too, so I thought that I would leave the changes here for consideration :)