Composer support

This pull request and the ones in the other Kirby repositories fix #46.
As I'm currently trying to make the Panel work, the pull requests to the boilerplate and the starterkit are still missing (Bastian, please see my mention on Twitter).

The pull requests add `composer.json` files to every code repository. All repositories are meant to be published on Packagist. Before this is possible, you need to:

1. Make sure the package names in the `composer.json` files are OK, as you can't change them later.
2. Create tags on every repository with the name `2.0.0-beta2`. This also applies to the patternlab, the forum etc. If you plan to call them `1.0.0`, please change the `require` options in all `composer.json` files to match the new version.

One comment regarding the conception: All changes here are still allowing for use of submodules (with exception of the test suite, which only works with Composer now, see the pull request).
This is done by trying to load the CMS or Toolkit in either `vendor` directories or the current paths.

Installing Kirby using Composer can be done by

1. Downloading a ZIP of *or* cloning the starterkit or bootstrap
2. Running `composer install --no-dev`
3. Done