Restructure background script (#70)

Splits some background script functionality into separate modules:
 - Receiver selector handling is moved to ./SelectorManager.
 - Status bridge handling is moved to ./StatusManager.
 - Menu creation and updates are handled in ./createMenus.
 - Shim creation is handled in ./createShim.

TypedEventTarget allows EventTarget-derived classes to export typed events.

Options type definition is moved to ./lib/options, module assumes more responsibility for update handling and provides a "changed" event.

Private cast._requestSession method allows bypassing receiver selector.

ext/src/manifest.json

@@ -27,6 +27,15 @@
         "scripts": [ "main.js" ]
     }
 
+  , "content_scripts": [
+        {
+            "all_frames": true
+          , "js": [ "shim/content.js" ]
+          , "matches": [ "<all_urls>" ]
+          , "run_at": "document_start"
+        }
+    ]
+
   , "content_security_policy": "CONTENT_SECURITY_POLICY"
   , "default_locale": "en"
   , "manifest_version": 2