Enabling coveralls.io

This tries to implement test coverage for coveralls.io. https://github.com/sass/libsass/issues/155

Work in progress as I have to test if this pull request compiles via travis ci :)
Once everything is working correctly, results should appear on coveralls.io.
This one should be `enable_coverage="$enableval"`. You should be able to pass `--disable-coverage`, or even `--enable-coverage=no`.