Add support for http-client

Hi, thanks for this package!

I would be great if you could add support for using psr/http-client instead of hardcoded curl calls for duing the http calls as that would make it a lot easier to write tests of code that depends on this phpcent.

Either that or at least making the request and send methods protected so that it is possible to extend them and provide our own implementation of the http handling.

Regards,
Tarjei