Add 'gdk' command wrapper and gem

Combine make and ./run into gdk. Can be invoked as gdk, e.g. gdk run db.

This does not remove make and ./run, it just adds an integrated wrapper.

Installation become:

gem install gitlab-development-kit
gdk init # clones repo
cd gitlab-development-kit
gdk install gitlab_repo=https://bla

doc/howto/browse.md

@@ -5,7 +5,7 @@ quickest way to do it.
 ```
 cd gitlab-development-kit
-./run
+gdk run
 ```
 Open [localhost:3000](http://localhost:3000) in your web browser. It