Merge pull request #51 from l/translation_ja

doc: update Japanese translations, add Japanese version of relay protocol
doc: fix typo in Makefile.am for Japanese docs
doc: build relay protocol before developer's guide
doc: update Japanese auto-generated files
core: add hidden buffers, add options hide/unhide in command /buffer
relay: add messages "_buffer_hidden" and "_buffer_unhidden"
doc: add "version >= 0.4.4" for messages _buffer_{hidden|unhidden} (relay protocol)
core: fix restoration of core buffer properties after /upgrade
core: add option -all for command /buffer hide|unhide
doc: add missing buffer properties in plugin API reference (hidden, zoomed, filter)
core: update German translation
doc: update German auto-generated files
debian: add Japanese version of relay protocol doc
xfer: fix help of option xfer.file.convert_spaces
trigger: remove obsolete comment
core: add option "-current" in command /print
trigger: interpret escaped chars in regex
irc: add tag with host ("host_xxx") in IRC messages displayed
trigger: add variable "tg_tag_host" (from tag "host_xxx" in message)
core: add tag "host_xxx" in /help filter