Add support for Product.not_for_sale flag.

This involved a couple of ancillary changes:

* The price field renderer will not display a price for products marked not
  for sale.

* The "grid" class now allows specifying a custom callable to provide
  additional CSS class for table rows.

* The products grid uses this to add a "not-for-sale" class to table rows
  for products which are marked thusly.

tailbone/grids/core.py

@@ -61,6 +61,9 @@ class Grid(Object):
     delete_route_name = None
     delete_route_kwargs = None
 
+    # Set this to a callable to allow ad-hoc row class additions.
+    extra_row_class = None
+
     def __init__(self, request, **kwargs):
         kwargs.setdefault('fields', OrderedDict())
         kwargs.setdefault('column_titles', {})
@@ -118,6 +121,20 @@ class Grid(Object):
         attrs = self.row_attrs(row, i)
         return format_attrs(**attrs)
 
+    def row_attrs(self, row, i):
+        return {'class_': self.get_row_class(row, i)}
+
+    def get_row_class(self, row, i):
+        class_ = self.default_row_class(row, i)
+        if callable(self.extra_row_class):
+            extra = self.extra_row_class(row, i)
+            if extra:
+                class_ = '{0} {1}'.format(class_, extra)
+        return class_
+
+    def default_row_class(self, row, i):
+        return 'odd' if i % 2 else 'even'
+
     def iter_fields(self):
         return self.fields.itervalues()
 
@@ -130,7 +147,3 @@ class Grid(Object):
 
     def render_field(self, field):
         raise NotImplementedError
-
-    def row_attrs(self, row, i):
-        attrs = {'class_': 'odd' if i % 2 else 'even'}
-        return attrs