client::Response is a struct
Add log on curl initialization
Improve ThreadPool
Workaround a possible asio bug

* Sometimes it seems that the deadline_timer is called with an Unknown
  error, which was ignored by the code. Now, we only ignore aborted
  error.
* Name ThreadPool
* Add curl_global_cleanup call
Add the possibility to cancel an async operation
Fix properly how connections should be handled in curl...
Add an usafe way of appending data to url