Fixes build to report correctly to coveralls.io

Do _not_ merge this before https://github.com/sass/sassc/pull/70 is merged!

This adds some major changes the travis ci and build chain. Create care has been taken to not break any existing setups. Thanks to @QuLogic for his groundwork with the autotools build (from where do you have this `non-existent-file-to-force-CXX-linking.cxx` voodoo?).

I have tested it on a local branch and you may check out the [coverage results](https://coveralls.io/builds/1294127).

I have left in some debug stuff which can/should be disabled at some point (indicated by comments in `.travis.yml`). It doesn't seem to hurt much, so I decided to leave it in for now. Coverage reports are correctly generated and reported for all variations (autotools=[yes/no], cxx=[g++/clang++]).

This pull request is expected to break on travis-ci since the needed PR for sassc is not merged yet. @hcatlin maybe you could restart that task on travis ci once the sassc PR is merged!