Allow one to specify timeout options when constructing PaymentClient

[jasonheecs]

Hi,

This PR adds a couple of config options (connection_time_out and request_time_out) when constructing PaymentClient. This will allow one to adjust the curl timeout, like so:

$client = new PaymentClient([
    'access_token' => "your access token",
    'environment' => "sandbox", //  or 'environment' => "production"
    'connection_time_out' => 200,
    'request_time_out' => 200
]);

Fixes #17