core: logging refactoring, first step: - added a core.log module - 3 backends can be used: basic, standard (python's logging module) or twisted - colors can be used - the module has been made to be used by frontends, it should work in exotic environments like pyjamas - logging basic configuration is now made in sat.tac - core.log configuration is inspired from python standard logging, and use it when possible - getLogger should be used the same way as for standard logging