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.
I added an SCM rockspec in 3d7571d2526dd3bbe784433bdd1659fd30426836, so the `.in` template file shouldn't be required now.