Entity field access condition 2281113b

Updated version of my previous patch;  I've changed branches due to some very bad problems with "git rebase".

I'm aware I have two commits that appear here.  That's the problem -- I can't get git to go lower without pulling in commits I don't own.  If you folks can tell me how to fix this, much thanks.

The patch creates the plugin and test for the issue.
> Don't worry so much about the test! All I want for that at this point is for it to show me that the plugin itself works. It doesn't have to be perfect because we are going to replace it with a unit test really soon. If you feel comfortable with unit tests, you can take a look at our other Condition plugin unit tests in rules/tests/src/Conditions (take the latest checkout from 8.x-3.x to see a set of which I already converted).
> 
> Really not required though. We can work through this together if you like (using simpletest) or you can give Unit tests a shot (which I am going to help you with too then). The existing unit tests should give you a good idea of what's going on.
> 
> —
> Reply to this email directly or view it on GitHub.
> 

My mail server was down since Thursday, so I just saw this.

Too late! :-)  I kept at it, and I have a non-trivial test working.  Runs nice and green, actually tests both of the main cases.

I'll see about getting my tree fixed.  If I can't, I'll just create a new branch and copy the two files over.  This will guarantee that the tree is clean.

Rob