Scripting support for articles and basic article properties

Also, added support for accessing feeds directly from the top level
container, essentially skipping account as a hierarchy level.

With this change, a script like

tell app “Evergreen”
   title of every article of feed "Six Colors" where read is true
end tell

produces the expected result.

Evergreen/Scriptability/Article+Scriptability.swift

@@ -0,0 +1,111 @@
+//
+//  Article+Scriptability.swift
+//  Evergreen
+//
+//  Created by Olof Hellman on 1/23/18.
+//  Copyright © 2018 Olof Hellman. All rights reserved.
+//
+
+import Foundation
+import Account
+import Data
+
+@objc(ScriptableArticle)
+class ScriptableArticle: NSObject, UniqueIdScriptingObject {
+
+    let article:Article
+    let container:ScriptingObjectContainer
+    
+    init (_ article:Article, container:ScriptingObjectContainer) {
+        self.article = article
+        self.container = container
+    }
+
+    @objc(objectSpecifier)
+    override var objectSpecifier: NSScriptObjectSpecifier? {
+        let scriptObjectSpecifier = self.container.makeFormUniqueIDScriptObjectSpecifier(forObject:self)
+        return (scriptObjectSpecifier)
+    }
+
+    // MARK: --- ScriptingObject protocol ---
+
+    var scriptingKey: String {
+        return "articles"
+    }
+
+    // MARK: --- UniqueIdScriptingObject protocol ---
+
+    // articles have id in the Evergreen database and id in the feed
+    // article.uniqueID here is the feed unique id
+
+    var scriptingUniqueId:Any {
+        return article.uniqueID
+    }
+    
+    // MARK: --- Scriptable properties ---
+    
+    @objc(url)
+    var url:String  {
+        return article.url ?? ""
+    }
+    
+    @objc(uniqueId)
+    var uniqueId:String  {
+        return article.uniqueID
+    }
+    
+    @objc(title)
+    var title:String  {
+        return article.title ?? ""
+    }
+
+    @objc(contents)
+    var contents:String  {
+        return article.contentText ?? ""
+    }
+
+    @objc(html)
+    var html:String  {
+        return article.contentHTML ?? ""
+    }
+
+    @objc(summary)
+    var summary:String  {
+        return article.summary ?? ""
+    }
+
+    @objc(datePublished)
+    var datePublished:Date?  {
+        return article.datePublished
+    }
+
+    @objc(dateModified)
+    var dateModified:Date?  {
+        return article.dateModified
+    }
+
+    @objc(dateArrived)
+    var dateArrived:Date  {
+        return article.status.dateArrived
+    }
+
+    @objc(read)
+    var read:Bool  {
+        return article.status.boolStatus(forKey:.read)
+    }
+
+    @objc(starred)
+    var starred:Bool  {
+        return article.status.boolStatus(forKey:.starred)
+    }
+
+    @objc(deleted)
+    var deleted:Bool  {
+        return article.status.boolStatus(forKey:.userDeleted)
+    }
+
+    @objc(imageURL)
+    var imageURL:String  {
+        return article.imageURL ?? ""
+    }
+}