Chefdk update

Update the plugin to work with ChefDK / Berks 3 and latest Vagrant version. Also rename to "vagrant-toplevel-cookbooks" (see README)

Summary of changes: 
* several **breaking changes**:
   * rename plugin to "vagrant-toplevel-cookbooks"
   * update to berkshelf 3 for [ChefDK](http://www.getchef.com/downloads/chef-dk) (berkshelf 2 is no longer supported)
   * remove berkshelf gem dependency (`berks` must be on the `$PATH`)
 * update development dependencies to latest vagrant 1.6 and adapt Vagrantfile samples