Add logging capabilities

[tanjeff]

The AgentXcpp should be able to log things, especially because it is hard/inconvenient to inform the user of the library about errors during asynchronous operation (e.g. in the master_proxy::receive() function).

However, agentXcpp should not rely on a certain logging library. It is up to the user to decide how to implement logging. Maybe the user can provide stream objects to the library which can be implemented as needed. Also, the logging feature should be completely optional (i.e. no logging by default).