Anchor related_url to beginning of request path

Closes #22.

menu_generator/menu.py

@@ -91,14 +91,14 @@ def _is_selected(self, item_dict):
         If related URLS are given, it also returns true if one of the related URLS is part of path.
         """
         url = self._get_url(item_dict)
-        if self._is_root(item_dict) and url in self.path:
+        if self._is_root(item_dict) and self.path.startswith(url):
             return True
         elif url == self.path:
             return True
         else:
             # Go through all related URLs and test 
             for related_url in self._get_related_urls(item_dict):
-                if related_url in self.path:
+                if self.path.startswith(related_url):
                     return True
         return False