use TARGET_URI if given, otherwise TARGET_URIS_FILE

@zeroSteiner,
Let me know what you think of this arrangement.  It's a little more in line with some default behaviors elsewhere in the framework.  Basically it registers TARGET_URI and TARGET_URIS_FILE.  If TARGET_URI is set, then it's treated as a single URI.  If TARGET_URI is not set, then TARGET_URIS_FILE is treated as the old TARGETURIS was treated.  Finally, if neither are set, an error is raised.  If you like it, merge this PR and your PR will automatically get updated and then I will land it.  Or, you can merge the PR and make changes of your own and your original PR will get updated still.  Or, if you think my PR is crap, just comment here and we'll work something out.