Rename offer migration to ensure number uniqueness

This will cause a few problems for sites that have already applied this
migration, but I think it's better in the long-run to have the
migrations use a unique number.
Allow Sphinx==1.2
Document migrations in 0.6 release notes
Reset page number when building facet URLs

It doesn't make sense to carry it through to the constructed URLs.

Fixes #1114
Merge branch 'master' of https://github.com/tangentlabs/django-oscar