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.
I like the comment.

I have two small change requests.

(1) Can you indent the commented line so that we avoid any YAML parse errors? (Different parsers seem to have different rules, but in general the indentation should match the line it's commenting).

(2) Why are we using `after_success`? It seems like this will end up running the tests twice. Or is that the intention? I'm thinking more along the lines of:

```
script:
  - mvn clean test jacoco:report
after_success:
  # the coveralls:report goal is responsible for updating the coveralls.io stats once the build completes
  - mvn coveralls:report
```

Or is the `clean test` required when using coveralls? If so, could we just run it all inside a single command in the script:

```
script:
  # the coveralls:report goal is responsible for updating the coveralls.io stats once the build completes
  - mvn clean test jacoco:report coveralls:report
```

Or perhaps the `after_success` is only run once and not for every time through the matrix. In which case, I see why it is done this way.