Add measurement coverage with coveralls

I am adding test coverage to dnsruby.

The test coverage shows how many percentage of lines are covered with tests.
It is a kind of metrics.

You can see the metrics if you have a [coveralls](https://coveralls.io/) account and enable this repository.
[coveralls page](https://coveralls.io/builds/1416683)
[coverage/index.html](https://dl.dropboxusercontent.com/u/12026408/dnsruby_coverage/index.html)(which are not commited)

It seems tests should be added more.
![dnsruby_coverage_image](https://cloud.githubusercontent.com/assets/1556477/4875890/622ffe02-62ac-11e4-9e62-0377c2902c38.png)
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