Rather than simply retrying after 503 errors, retry after all error
responses since we have also been seeing some spurious 400 responses
from Google.

This causes Lookup groups and their members to be synchronised with
groups in Google. As for people, there are various configuration
options to control which groups to synchronise, what settings should
be applied to them, and to enforce limits on the number of changes the
script will perform.

The group email and display name in Google are based on the Lookup
groupID and groupName, noting that the email is the group's unique
identifier in Google:

email = {groupID}@{groups_domain}
name = {groupName}{group_name_suffix}

where groups_domain is a configurable secondary domain for groups in
Google, and group_name_suffix is a configurable suffix appended to
group names in Google.

Provide an initial implementation f the tool and README documenting how
to install, configure and run it.

Closes #1