add coveralls auto tool to json-c

[dota17]

This PR adds the coveralls auto tool to json-c. Coveralls is a web service to help you track your code coverage over time, and ensure that all your new code is fully covered.

I have added it in dota17/json-c and showed a PR submission example in here. We can see the json-c's code coverage in README.md and get the code coverage report from coveralls automatically when you submit a PR. It is a useful and helpful tool.

Following are the configuration steps：

1. Sign in the https://coveralls.io/ with github account and open the switch of json-c/json-c in ADD REPO .
2. configure file:
   
   
   
   note: Compact means that you will get a neat and clear report.
3. Merge this pr.
4. Get and add the badge into README.md
   
   These steps are simple and coveralls is an interesting and useful tool. You will like it. （‐＾▽＾‐）

[dota17]

And, we should choose suitable files to report. In here, I have chosen the most of the .c files and partial .h files.

[hawicz]

Showing test coverage sounds like a good idea. However, is it really worth it to pull in another separate service to do that? It seems that we should be able to get the same percent coverage simple out of gcov and/or lcov.

[dota17]

I have modified the .travis.yml. And I think it is necessary and worthy to run it in a separate service. It won't affect other services.

[dota17]

Because json-c/json-c repository does not have the usage rights of coveralls, this PR could not pass. I suggest you give the permission to Coveralls as the steps shown in PR description.（‐＾▽＾‐） @hawicz

[dota17]

@hawicz I have modified the .travis.yml. The CI couldn't pass and maybe need you to give the permission to Coveralls. Coveralls has provided a lot of convenience for community submission and code testing. It really is a good tool, ≧◉◡◉≦

[dota17]

@hawicz I have opened the permissions of Coveralls and the CI has passed. Do you have any other opinions on this？

[dota17]

This PR is not associated with the core code, so I will merge it. We can try it out. If it is not suitable, we can revise/delete it.