Replaced multiple curl_setopt with a curl_setopt_array

Here are the points of my argument for this PR.

First, the requirement of using `curl_setopt_array` is satisfied. `json_decode` needs PHP 5.2.0 and higher and `curl_setopt_array` needs PHP 5.1.3 and higher.

Second,  the logic of using `curl_setopt_array` is more clear than that of using `curl_setopt`. `curl_setopt_array` promote the separation of the concerns about preparing the options and the initialization of the curl.

Third, `curl_setopt_array` is slightly faster than mutiple `curl_setopt`. Here is my benchmark code.

https://gist.github.com/3330620

Lastly, I deleted the lines of initialzing `$this->http_info` and using `array_merge`. These lines do nothing.

```php
$this->http_info = array();
$this->http_info = array_merge($this->http_info, curl_getinfo($ci));
```

Thanks.