lua: allow sending lua commands via dbus

use org.darktable.service.Remote.Lua to send a lua string
lua: fix a typo in usermanual
dt_histogram_worker: fix memleak
Levels, exposure: better commit_params logic (in case of bad histogram)

Anything broken?
IOP Exposure: Deflicker: s/"thumbnail"/"pre-processed image"/
IOP Exposure: Deflicker: show mode switch only when dt_image_is_raw()
lua : return nil instead of NAN on latitude/longitude of images when not set
usermanual: several minor additions/corrections (plus xml source pretty-printing)
usermanual: update to the exposure module
lua usermanual : rework function synopsis to be more docbook friendly
usermanual: add missing hint in exposure module
Italian translation updated
Merge pull request #590 from bartokk/master

Italian translation updated
lua : fix tags reporting all images instead of the ones attached to them
lua : gui functions are only available when gui is enabled
allow to pass a lua_State to dt_init

This lua_State will be used if provided.
if set to NULL, a new one will be created instead
lua : fix fomat.write_image not working properly
lua : allow darktable to be a pure lua module

you can now call DT from any lua script without an actual
instance of DT running
rawspeed: update to current develop branch
rawspeed: remove some cruft