switch to the logging module

Instead of rolling our own log/warn/debug interface like right now, we should reuse the logging module, which will enable us to have global logging access.

Right now, we can't log in the gpg module for example, because it doesn't have access to the ui object. gpg, like all other modules, should use logging and the ui should just hook a Handler adequate for its needs.

For example, in the GTK UI, this could be a status bar. An archive of all logging could also be kept accessible in memory for consultation in a specific window, see #44 (closed) for the help dialogs implementation.