Fix #4 for older ConfigParser versions

Python 2.6 has an older version of ConfigParser, which requires us to change
the dict_type.
release 0.1.6, remove data_files
Fix #5, show empty values for non-existing users
release 0.2
Add cache of whether a user_id exists

Multiple calls of check_users_exist() with the same user won't result in
multiple requests now.
release 0.2.1