Attempt to add Disqus comment count

Per instructions at http://help.disqus.com/customer/portal/articles/565624

See #338 

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### remaining tasks

 -  [x] @briancavalier to debug comment counts disappearing via https per https://github.com/spring-io/sagan/pull/339#issuecomment-40068925
 - [x] @briancavalier to get 'comments' css in place per https://github.com/spring-io/sagan/pull/339#commitcomment-5968855
 - [x] @dararith (or @lshannonPivotal) to design 'comment' icon and advise on any other styling matters per https://github.com/spring-io/sagan/pull/339#commitcomment-5968974
 - [x] Shannon to update sprite with new comment icon per https://github.com/spring-io/sagan/pull/339#commitcomment-5990978