title: Convert timestamp to a specified timezone offset
body: <p>How can I convert a long timestamp, e.g. <code>2556072000000L</code>, to a String in a specified timezone offset, e.g. <code>+08:30</code>? I looked into Joda methods but didn't find a solution.
  Similarly, how can I convert a String timestamp, e.g. <code>2050-12-31T04:00:00Z</code>, to a String in a specified timezone offset, e.g. <code>+08:30</code>?
  <code>2050-12-31T12:30:00+08:30</code> in the specified offset is expected.</p>
  <p>Parsing <code>+08:30</code> into three parts: <code>+</code>, hour offset (<code>8</code>) and minute offset (<code>30</code>), add the offset to the <code>timestamp</code> or <code>dateTime</code> could be a solution. However, I'm wondering whether there's already a solution so that I needn't reinvent wheels?</p>
  <pre><code>/*
   * timestamp: 2556072000000L (2050-12-31T04:00:00Z)
   * offset: +08:30
   * return: 2050-12-31T12:30:00+08:30
   */
  String toOffset(long timestamp, String offset);
  
  /*
   * dateTime: 2050-12-31T04:00:00Z
   * offset: +08:30
   * return: 2050-12-31T12:30:00+08:30
   */
  String toOffset(String dateTime, String offset);
  </code></pre>