doc: add tags for missing translations in Japanese developer's guide
core: remove obsolete WeeChat version in weechat.pot
doc: add non-official support channel #weechat-de in user's guide
core: add a link to user's guide or WeeChat site for contacting developers
irc: fix translation of CTCP PING reply (closes #137)
irc: remove unneeded variable "seconds"
core: fix translation of message displayed after /upgrade
core: update German translations
core: add link to user's guide in AUTHORS
core: add anchor "#support" for link to user's guide in AUTHORS
doc: add missing files sec.conf and exec.conf in man page