Add term, user, and node reference fields and per bundle properties

I didn't add per bundle properties, if you want that, I think you should do something like the following code :

$info[$target]['properties']['bundles'][$bundle].

However I added support for other entity types, it should also works for commerce_product_reference etc.

It seems like jpstrikesback original request was for source specific backreference fields where as jsacksick suggestion in #4 would address target specific backreference fields. It just so happens that I created target specific patch. This will help prevent backreference fields from being available on bundles they shouldn't be. I did not tackle the original source specific request as that would require an update hook to rename fields and moving existing data. Patch attached.

Committed, thanks!