Further progress on refining the REST server side code.

1) Deal in fully qualified URL resources through the rest
   interface.  All rest methods are now passed the complete url in
   request->url.

2) Create rest::resolve() which lets individual resource definition
   code convert a full url into the appropriate matching resource.
   Implement gallery_rest::resolve() and tag_rest::resolve()

3) Reimplement tag_rest's get() and post() methods.  They're much
   simpler now.

4) Implement the tags_rest helper which supports working with the
   entire tags collection.

modules/rest/helpers/rest.php

@@ -51,4 +51,24 @@ class rest_Core {
   static function send_headers($exception) {
     header("HTTP/1.1 " . $exception->getCode() . " " . $exception->getMessage());
   }
+
+  /**
+   * Convert a REST url into an object.
+   * Eg: "http://example.com/gallery3/index.php/rest/gallery/Family/Wedding" -> Item_Model
+   *
+   * @param string  the fully qualified REST url
+   * @return mixed  the corresponding object (usually a model of some kind)
+   */
+  static function resolve($url) {
+    $components = explode("/", substr($url, strlen(url::abs_site("rest"))), 3);
+
+    // The first component will be empty because of the slash between "rest" and the
+    // resource type.
+    $class = "$components[1]_rest";
+    if (!method_exists($class, "resolve")) {
+      throw new Kohana_404_Exception($url);
+    }
+
+    return call_user_func(array($class, "resolve"), !empty($components[2]) ? $components[2] : null);
+  }
 }