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

@@ -15,7 +15,7 @@ def main
       return false
     end
-    cmd = %W(git clone -b gdk-cli https://gitlab.com/gitlab-org/gitlab-development-kit.git)
+    cmd = %W(git clone https://gitlab.com/gitlab-org/gitlab-development-kit.git)
     cmd << ARGV[1] if ARGV.count == 2
     system(*cmd)
   else