Add `seek` method to `File` class

This method is necessary for random-access streams.  It will also be
helpful in the future when implementing the [IO/A spec][1].

[1]: http://wiki.commonjs.org/wiki/IO/A#Instance_Methods

http://code.google.com/p/phantomjs/issues/detail?id=937

src/filesystem.cpp

@@ -110,6 +110,15 @@ bool File::write(const QString &data)
     }
 }
 
+bool File::seek(const qint64 pos)
+{
+    if (m_fileStream) {
+        return m_fileStream->seek(pos);
+    } else {
+        return m_file->seek(pos);
+    }
+}
+
 QString File::readLine()
 {
     if ( !m_file->isReadable() ) {

src/filesystem.h

@@ -52,6 +52,8 @@ public slots:
     QString read();
     bool write(const QString &data);
 
+    bool seek(const qint64 pos);
+
     QString readLine();
     bool writeLine(const QString &data);