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.
Yes, good point. I guess the "use distro vs. build from scratch" discussion is dateless ([and I had it again, recently](https://github.com/openresty/test-nginx/pull/17#issuecomment-51683559)). The key point is to find a good balance. Nowadays I would agree, that lua is as much a dependency as a C compiler and rather refrain from building it "from scratch".
So I will just drop "lint" from the build. We could add feature detection later (does ```luarocks``` support ```lint``?).