v2ex_login_with_ASI

- (void)doLogin{
    // 发送登陆表单
    NSURL *url = [NSURL URLWithString:@"http://www.v2ex.com/signin"];
    loginRequest = [ASIFormDataRequest requestWithURL:url];
    [loginRequest addPostValue:username forKey:@"u"];
    [loginRequest addPostValue:password forKey:@"p"];
    [loginRequest addPostValue:once forKey:@"once"];
    [loginRequest addPostValue:@"/" forKey:@"next"];
    [loginRequest setUserAgentString:@"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/34.0.1847.116 Safari/537.36"];
    [loginRequest setShouldRedirect:NO];
    [loginRequest setPostFormat:ASIURLEncodedPostFormat];
    [loginRequest addRequestHeader:@"Content-Type" value:@"application/x-www-form-urlencoded"];
    [loginRequest startAsynchronous];
    return;
}