Debug logging

[zplesac]

We should add some logging to the library, as it's extremely hard to debug it when we have some problems.

I suggest that we add LogUtils like in Goldfinger and log at least:

• the response from the URL
• how is this response parsed and processed in the lib, especially when we receive ONCE event - we'll we notify the callback listener or not.

Debugging should be enabled/disabled with a global configuration class.