Feeds new mapping configuration for entity reference mapping keys

Hi,

I know that #1616680: Handle more mapping keys in Feeds integration is somewhat freshly committed, but I did some work on this before I knew that it had already been done in dev (silly me, I should have checked the issue queue better). My target mapper was almost the same, actually, except that I used the new(ish?) mapper configuration form as a means to select how to match entity references.

It only took about 15 minutes to merge the configuration form stuff in so I thought I would post it here and see what you guys think.

A note about the changes: I left some code in there that checks for the old method of parsing the match key from the target name for backwards compatibility. However, if this patch applied and there were target mappings using the old method, those mappings show up as Missing on the mapping configuration page.

Also, I set this patch specifically to not be tested as the feeds tests will need to be updated to account for the changes. I didn't want to update the tests until I knew if it was something you'd like to consider for entityreference.

Thanks for your time!

drclaw