Add blacklist to notifier

We want to ensure that some users never get emailed about whether their
data sets are out of date.

This sets up a file called blacklist.json which will be used inside of
the emailer instance to block emails being sent to anyone in that list.

This ignores case difference between the blacklist and the email address
that is being sent to.
Added failing test for multiple emails being sent
Let the emailer send an email for people not in the blacklist.

Before if there was an email in the blacklist no email was sent.

This now just removes that email from the list of emails to send and carries on.
Log if no emails in list so no email to be sent.
Merge pull request #31 from alphagov/feature/blacklist

Add blacklist to notifier