Add Resource::open() method to make URL creation simpler

README.md

@@ -75,7 +75,7 @@ $file= new File('large.txt');
 $file->open(File::READ);
 
 try {
-  $transfer= $s3->open('PUT', 'target/'.$file->filename, [
+  $transfer= $s3->resource('target/{0}', [$file->filename])->open('PUT', [
     'x-amz-content-sha256' => SignatureV4::UNSIGNED, // Or calculate from file
     'Content-Type'         => 'text/plain',
     'Content-Length'       => $file->size(),

src/main/php/com/amazon/aws/api/Resource.class.php

@@ -74,4 +74,15 @@ public function transmit($payload, $type= 'application/json') {
       $this->serialize($this->marshalling->marshal($payload), $type)
     );
   }
+
+  /**
+   * Opens a request and returns a `Transfer` instance for writing data to
+   *
+   * @param  string $method
+   * @param  [:string] $headers
+   * @return com.amazon.aws.api.Transfer
+   */
+  public function open(string $method, array $headers) {
+    return $this->endpoint->open($method, $this->target, $headers);
+  }
 }