travis CI integration

Please review. Note that there are lots of commits because travis builds only trigger on git push.
The basic setup is taken from https://github.com/moteus/lua-travis-example, augmented with a pkg-config file based on http://www.linuxfromscratch.org/blfs/view/svn/general/lua.html.
All scripts are transformed to comply with the indentation of this repo (4 spaces per level).

If you decide to integrate this you need to change the travis links to your own account, e.g.
https://travis-ci.org/istr/lua-gumbo.png?branch=master -> https://travis-ci.org/craigbarnes/lua-gumbo.png?branch=master.
As for lua from packages this seems to work. [The pre-packaged luarocks version, however, is too old](https://travis-ci.org/istr/lua-gumbo/jobs/42646635) to support the ```lint``` command. And using apt-get might not work if we plan to [extend to travis mac environment](http://docs.travis-ci.com/user/osx-ci-environment/). Thus the only benefit using distribution packages would be, that ubuntu users can be sure it integrates with packages from their distribution (which it currently does not, because a recent luarocks version is needed).