Added configuration for coveralls.io

This PR is meant to close the issue #107 adding coverage data with coveralls.io.
For @mojavelinux advise I added a coment on the _.travis.yml_ file and created a wiki page descriving the process.
https://github.com/abelsromero/asciidoctor-maven-plugin/wiki/Coveralls.io-configuration
If everythingt is fine with the document I'll migrate it to the main ascidoctor's wiki.

PS: once is merged I'll edit the badge on the readme.