Paginator._get_page hook

This allows for Paginator subclasses to easily override the Page class
that gets used.

Took the opportunity to also do some non-invasive PEP8 tidying of the
paginator module.

tests/regressiontests/pagination/custom.py

@@ -0,0 +1,20 @@
+from django.core.paginator import Paginator, Page
+
+
+class ValidAdjacentNumsPage(Page):
+
+    def next_page_number(self):
+        if not self.has_next():
+            return None
+        return super(ValidAdjacentNumsPage, self).next_page_number()
+
+    def previous_page_number(self):
+        if not self.has_previous():
+            return None
+        return super(ValidAdjacentNumsPage, self).previous_page_number()
+
+
+class ValidAdjacentNumsPaginator(Paginator):
+
+    def _get_page(self, *args, **kwargs):
+        return ValidAdjacentNumsPage(*args, **kwargs)