Building extension files

Adds possibility to build extension files (Chrome and Safari) from
command line.

To run from the project directory:
python tools/build.py [meta]

If the optional `meta` argument is set, then only the manifest and
language files are uptated.
Without that everything is being built (extension files too) into the
`dist/build/version_number` folder.

For Chrome there will be two files, a crx, and a .zip file which
includes the key.pem private key (so this must not be shared,
it's just a bit help for publishing it to the Chrome Web Store).

Beside the extension files, update-files are generated too (for self
hosting - Safari needs it).

src/js/vapi-background.js

@@ -1,3 +1,4 @@
+// » header
 /* global SafariBrowserTab, Services, XPCOMUtils */
 // for background page only
 
@@ -5,8 +6,10 @@
 'use strict';
 
 self.vAPI = self.vAPI || {};
+// «
 
 if (self.chrome) {
+    // » crx
     var chrome = self.chrome;
 
     vAPI.chrome = true;
@@ -272,7 +275,9 @@ if (self.chrome) {
             chrome.contextMenus.remove(this.menuId);
         }
     };
+    // «
 } else if (self.safari) {
+    // » safariextz
     vAPI.safari = true;
 
     // addContentScriptFromURL allows whitelisting,
@@ -888,9 +893,13 @@ if (self.chrome) {
             this.onContextMenuCommand = null;
         }
     };
+    // «
 }
 
+// » footer
+
 if (!self.chrome) {
     self.chrome = { runtime: { lastError: null } };
 }
 })();
+// «