Remove dependency on gconf

Created by: GeraldJansen

• completely remove gconf dependency (also in waf scripts)
• save/load configuration in xdg_config_home, normally ~/.config/hamster-time-tracker/hamster.json
• for now the file is accessed on every conf.set() and conf.get() call (meaning that external modifications will be picked up immediately)

Related to #64 (closed). Perhaps a complex migration is not necessary.