Change production URL to SendCloud's mockserver for testing

[arnausaima]

To be able to do unit testing and/or for development porposes, SendCloud API allows requests to be made to a mockserver instead of production server. It would be interesting to make $apiUrl of Connection class editable in constructor.

[casperbakker]

If you like support for that, please create a PR and I will take a look.

[arnausaima]

#59