Added 'dig-style pagination'

Created additional pagination style `ellipses_pagination` which emulates
dig-style pagination, see issue #638. Instead of links only to previous
and next page, the list of page numbers is displayed. To prevent
displaying enormous number of page numbers, only couple of page numbers
are shown (first N, couple of pages around the current page, last N
pages). See `oscar/templatetags/ellipses_pagination.py` for algorithm
description.

The new pagination is not deployed as default pagination yet. You can
change that by changing partials to `ellipses_pagination` instead of
`pagination`.