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.
Yeah, that's a good point. I agree we should test the luarocks build process properly for each commit. Let's just remove the `luarocks lint` step from the rockspec generator.

However, I'm still against building from source anything that's available via a package manager. The end result of building from source shouldn't be any better than what comes via a package, except for not having to maintain and support packaging scripts in this repo. What happens when there's a new Lua release? Do we really need to duplicate the effort that distro packagers have already done? Should we also write scripts for building `gcc`, `make` and `binutils` from source too?