Travis ci

As I’m on a roll right now…

I propose that [Travis-CI](https://travis-ci.org) integration is turned on for this project. This PR adds support for it (obviously also needs an owner of this repo/org to turn it on).

It uses [Justin Spahr-Summers’ Objective-C build scripts](https://github.com/jspahrsummers/objc-build-scripts) to command [xctool](https://github.com/facebook/xctool) which will, when asked, build the main project and also run any unit tests.

Try it locally with `./script/cibuild`.

I let the `bootstrap` script be left in as it then provides an explicit, compliant and future proof way of bootstrapping the project. If you feel it adds to much confusion (as the project is completely self-contained at the moment) I’ll remove it.