added HEAD request capabilities

sample usage: 

```java
TwitterRestClient.head("statuses/public_timeline.json", null, new AsyncHttpResponseHandler() {
     @Override
     public void onSuccess(int statusCode, Header[] headers, String content) {
          System.out.println("----header start----");
          for(final Header h : headers) {
               System.out.println(h.getName() + ":" + h.getValue());
          }
          System.out.println("----header finish----");
     }
});
```