added HEAD request capabilities

addresses issue https://github.com/loopj/android-async-http/issues/146

sample: 

added to TwitterRestClient as seen in the [docs](http://loopj.com/android-async-http/):

```java
import com.loopj.android.http.*;

public class TwitterRestClient {
  private static final String BASE_URL = "http://api.twitter.com/1/";

  private static AsyncHttpClient client = new AsyncHttpClient();

  //
  //new
  public static void head(final String url, final RequestParams params, final AsyncHttpResponseHandler responseHandler) {
      client.head(getAbsoluteUrl(url), params, responseHandler);
  }
  //new
  //

  public static void get(String url, RequestParams params, AsyncHttpResponseHandler responseHandler) {
      client.get(getAbsoluteUrl(url), params, responseHandler);
  }

  public static void post(String url, RequestParams params, AsyncHttpResponseHandler responseHandler) {
      client.post(getAbsoluteUrl(url), params, responseHandler);
  }

  private static String getAbsoluteUrl(String relativeUrl) {
      return BASE_URL + relativeUrl;
  }
}

```

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----");
     }
});
```