paginator should validate page and provide default

- use the standard paginator.validate_number method rather
  strict_postive_int.
- support optional paginator method, default_page_number, to get the default
  page number rather than hard-coding it to 1
- this allows supporting non-integer based pagination which can be an
  important performance tweak on extermely large datasets or high request
  loads
- relatively thorough unit tests of the changes