Refactor http request handling

[jonasschnelli]

Libcurl is used also for the Qt part right now. The reason for this is that we can use most core classes in a non QT cmd tool later.

HTTP Request handling should be refactored and there should be only one C++ class handling the request. Currently, dbb_gui.cpp (update checker) / libbitpaywalletclient.cpp (copay backend) and soon the ComServer will use curl and do not share the http request code.