Add per-rrset-comments
JSON API: Catch wrong container types

Avoids an assert/abort in rapidjson.
Travis: Add travis_retry for rubygems operations

As per http://blog.travis-ci.com/2013-05-20-network-timeouts-build-retries/
and a recommendation from @travis-ci.
Merge pull request #1301 from zeha/comments

Add per-rrset-comments