Add mailgun output

Add mailer to send email via the mailgun http api. This will allow users
to send emails alerts, without having to configure sendmail or some
equivalent.
Switch to clj-http and fix ns declaration
Add unit tests for mailgun output
Merge pull request #447 from dlobue/add_mailgun_output

Add mailgun output
Fix mailgun output unit test

Can't use a static string for the body stub as the timestamp generated
by the body function is based on the timezone of the machine it is run
on. Use the default body function instead.
Merge pull request #448 from dlobue/fix_mailgun_test

Fix mailgun output unit test