Limit page size to 100, for fetching Person records from NB API

apparently that is the effective limit..so must honor it

rattail_nationbuilder/importing/nationbuilder.py

@@ -103,7 +103,8 @@ class NationBuilderCachePersonImporter(FromNationBuilder, nationbuilder_importin
     ] + primary_address_fields
 
     def get_host_objects(self):
-        return self.nationbuilder.get_people(page_size=500)
+        return self.nationbuilder.get_people(page_size=100,
+                                             progress=self.progress)
 
     def normalize_host_object(self, person):
 

rattail_nationbuilder/nationbuilder/webapi.py

@@ -95,6 +95,10 @@ class NationBuilderWebAPI(object):
 
         https://apiexplorer.nationbuilder.com/nationbuilder#People
         """
+        # nb. found this limit in practice, but it is not documented?!
+        if page_size > 100:
+            raise ValueError("page_size cannot be more than 100")
+
         response = self.get('/api/v1/people/count')
         count = response.json()['people_count']
         pages = count // page_size