Fix sorting for raw_id objects

- Added common function to get querystring filters
- Excluded querystring parameters used for raw_id fields
- Version bump to 2.1.13
- Updated readme

sample_project/samples/admin.py

@@ -52,6 +52,12 @@ class WidgetAdmin(SortableAdmin):
 admin.site.register(Widget, WidgetAdmin)
 
 
+class CreditAdmin(SortableAdmin):
+    raw_id_fields = ('project',)
+
+admin.site.register(Credit, CreditAdmin)
+
+
 class CreditInline(SortableTabularInline):
     model = Credit
     extra = 1