Changes:
- build of doc now requires weechat-headless, translations and all plugins
- convert docgen.py to C
- remove `autogen_*` files from repository
- add command line parameter `--doc-gen` in `weechat-headless` to build
autogen files
- build .mo files with directories like the installed ones
(eg: "<lang>/LC_MESSAGES/weechat.mo")
- remove javascript chapter from user's guide