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 Job ID Name Coverage
  Test
passed #2625
Go-py2.7

00:01:35

passed #2626
Go-py3.4

00:01:47

passed #2627
Go-py3.5

00:01:41

passed #2628
Go-py3.6

00:03:24