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

bin/gdk

@@ -5,7 +5,7 @@ require 'fileutils'
 #
 # Note to contributors: this script must not change (much) because it is
 # installed outside the gitlab-development-kit repository with 'gem
-# install'.
+# install'. Edit lib/gdk.rb to define new commands.
 def main
   case ARGV.first