Composer support

See https://github.com/getkirby/kirby/pull/53 for further details.

Some information about this PR:

- The way to install and test the core has changed, so please update the `README` after this is merged.
- PHPUnit has been defined as dev-dependency, as it makes sure the PHPUnit version is the same everywhere this is tested.
- There are two ways you can run the tests:
  - Install the starterkit or boilerplate using `composer install` (no `--no-dev`) and then run `vendor/bin/test`. There is currently a problem getting the current path because of the symlink created by Composer, which could be fixed by using `readlink -f` in `test`, but this doesn't work on OS X. Solution: Run `vendor/kirby/tests/test` directly instead.
  - Download this repository, install a local PHPUnit using `composer install` and then run `./test`.

This makes it possible to test a Kirby installation or just the naked core.