Fauxton: Improved pagination

This is a new version of pagination in Fauxton using skip. It uses a
PagingCollection that has the main algorithm for pagination and exposes
a nice api.

This is an intermediate step as this is a much better pagination than we
have at the moment. However using just skip for pagination is not
optimal as there are two cases where skip pagination fails - For very
large skips and for when documents that a user have paginated past have
been deleted.

The next step once this has landed will be to add in a startkey_docid
pagination as well. The PagingCollection would then decided which method
to use to paginate for an index.