Add readTemplateSync() to open a mustache template

We'll be opening a template for the summary email stuff too, so we'll
want to reuse this use of fs.readFileSync().
Add basic logic for summary emails

This commit creates new templates for the weekly summary and uses
Emailer to send them to users.
Add stub module updates to email summary

moduleUpdates will (in future) contain an object for each module
that the summary email supports, of the form:

    {'title': 'Digital take-up', 'textUpdate': 'Changed by 12%'}
Test that one email is sent per recipient

Each recipient should receive their own summary email. Add a test
to ensure that email.send() is called once per person.
Merge pull request #36 from alphagov/used-shared-client-for-dashboard-data

Add summary email template
Summary emails - list module updates out
Support single time series modules for summary email

Reference JSON for single time series
Support grouped time series modules in summary email
Merge pull request #37 from alphagov/summary-email-content

Summary email content - list module updates
Update to work with latest dashboard JSON (formatted change is an object)
Merge pull request #40 from alphagov/kpi-summary

Update to work with latest dashboard JSON
Remove support for grouped timeseries in summary emails
Merge pull request #41 from alphagov/remove-grouped-timeseries

Remove support for grouped timeseries in summary emails