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.
Generally I would agree that people do not build the rock from git. So this raises the question what the travis ci check should be good for:
- check that users are able to install the rock from some external repo (if so, the travis ci check could be boiled down dramatically: just fetch and install the latest published rock from somewhere and run the test suite against that installation)
- check that the full build process (which includes building a rock) completely works on several platforms and lua flavours and show developers how these platforms are to be set up

I tend to support the latter (and consequently tend to explicitly install anything that is needed from sources), but ymmv.