Add the possibility of more logging

[wsw70]

It would be great if there was some more logging generated by the application.

Right now it is just the HTTP conversation that gets logged bus several what I belive to be key events are not:

• start of the program (with some info about the version etc.)
• client addition
• emails sent (an error is logged)
• changes in configuration

[vx3r]

The version is quite important, added in front and backend. Let me think about other logging. Basically with HTTP verb and URI you can guess what was the event and if the response is different from 200 error will be logged.

[lukasmrtvy]

+1 for structured log format ( json )