support for permalink and external url

Consolidate GetURL AppleEvent handling into AppDelegate+Scriptability
file
Add scripting access groups to sdef
Add exists command
Add ‘permalink’ and ‘external url’ properties to the article scripting
object
Add a unit test to verify the behavior of ‘current article’

EvergreenTests/ScriptingTests/scripts/testCurrentArticleIsNil.applescript

@@ -0,0 +1,23 @@
+-- this script   tests that it is possible to get the url property of the current article
+-- it uses system event accessibility scripting to set up the main window
+-- one needs to authorize scripting accessibility control in the System Preferences'
+--  Privacy and security pane
+
+try
+    tell application "Evergreen"
+        set shouldBeMissingValue to current article
+    end tell
+
+    --verify that the current article is in fact 'missing vcalue'
+    if shouldBeMissingValue is missing value then
+        set the_message to "passed tests"
+        set the_result to true
+    else
+        set the_message to "expected current article to be 'missing value'"
+        set the_result to false
+    end if
+on error message
+	return {test_result:false, script_result:message}
+end try
+
+return {test_result:the_result, script_result:the_message}