Add support for coveralls.io coverage reports

OK, it's true, those badges in the README might be a little obsession of mine. Nevertheless, it occurs more often than it should that some behaviour in the jekyll code is accidentally broken, yet no test goes :boom: like it should - because there is no test for that particular behaviour. In most cases it's detected before merge, but there are also cases that go into the next release and then :shit: goes down.

So, while it is not necessary to test every single line and a coverage of 100,000000 % is surely not necessary, some visible report on it can probably help to improve jekyll's (already awesome) testing suite. This PR adds support for coverage analysis during Travis-CI builds and loading them up to coveralls.io (provided @mojombo signs in there with his github account).

See https://coveralls.io/r/maul-esel/jekyll for the current reports of this branch.