Add menu item takes rel urls, offers new node.  #1958

When add menu item has an internal path that goes nowhere, it now offers to create a new node using that path.  I didn't want to add a new page for dealing with this, so the invalid path error message now presents a select box and an explanation.  Getting that form item to show up was trickier than expected.

While I was doing this I ran into a case where form errors weren't being returned.  Explicitly returned them as part of the ajax command already being sent.

This isn't quite finished yet.  It saves everything that needs to be added to the node form to the wizard cache, but I'm getting og_vocab errors on node/add, so I haven't gotten to deal with the rest of it yet.
Add menu item.  URL can create node.  #1958

The "url does not exist, would you like to create it" workflow works.  Nodes get their new paths and are created.

Needed an update hook to weight cp_menu over pathauto.

Mostly done, but still doing a little sanity checking.
add menu item.  changed functions used in checking existing path.  #1958

drupal_get_normal_path and drupal_valid_path were the right combo of core functions for checking if the provided path was in use or not.
Removed unnecessary purl prepend #1958
Merge pull request #1977 from openscholar/1958-add_menu_relative

1958 add menu - relative links and new nodes