Add vim modeline to all Python files

Uses four spaces for tab and shift widths, and turns on expansion of
tabs to spaces.

config.py

@@ -3,3 +3,5 @@ import os
 # Check if Solr connection information was provided in the environment
 SOLR_SERVER = os.environ.get('SOLR_SERVER', 'http://localhost:8080/solr')
 SOLR_CORE = os.environ.get('SOLR_CORE', 'statistics')
+
+# vim: set sw=4 ts=4 expandtab: