Add twitter metadata

- copied again from masonstrap

Closes #55
4 jobs for 55-twitter-cards in 3 minutes and 25 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed Go-py2.7 #2625

00:01:35

passed Go-py3.4 #2626

00:01:47

passed Go-py3.5 #2627

00:01:41

passed Go-py3.6 #2628

00:03:24