Add logging

[slaperche-scality]

In order to:

• see what's happen during a simulation
• ease the troubleshooting when things go awry

We should have a proper logging implemented in Quadiron.

After a quick look at the library available, I think we can go with easyloggingpp. It has minimal dependencies and a good feature set, amongst which:

• loggers can be setup from a configuration file
• loggers can be set independantly (normal case => enable simulator logging, disable DHT logging; debugging => enable tracing for the DHT module).