Let a grid have custom ajax data url

i.e. instead of assuming we should GET the current "partial" page, can use a
different URL altogether now

tailbone/grids/core.py

@@ -74,6 +74,7 @@ class Grid(object):
                  sortable=False, sorters={}, default_sortkey=None, default_sortdir='asc',
                  pageable=False, default_pagesize=20, default_page=1,
                  checkboxes=False, checked=None, main_actions=[], more_actions=[], delete_speedbump=False,
+                 ajax_data_url=None,
                  **kwargs):
 
         self.key = key
@@ -127,6 +128,13 @@ class Grid(object):
         self.more_actions = more_actions or []
         self.delete_speedbump = delete_speedbump
 
+        if ajax_data_url:
+            self.ajax_data_url = ajax_data_url
+        elif self.request:
+            self.ajax_data_url = self.request.current_route_url()
+        else:
+            self.ajax_data_url = ''
+
         self._whgrid_kwargs = kwargs
 
     def make_columns(self):