overhauled config gui navigation, fixed spacing in general page, renamed comments
removed manual check interval from config dialog (still configurable in the settings file)
set default manual mail check interval to 10 mins
renamed mailnag_config script to mailnag-config, edited manpage
renamed [general] settings section to [core]
enforce program termination if cleanup takes too long or if therere are remaining active threads
fixed SIGTERM handler

see https://bugzilla.gnome.org/show_bug.cgi?id=689208 for details