Rename conveyor_belt to stream.
Fix a bug when taking damage without shield
Update the license file, adding sources
Added new sounds from opengameart, to check.

Also changed the warp sound to make it free, and updated the license file.
Added a pop effect on poison gaz

Poison gaz must be recalibrated by the way : too hard to avoid it...
Changed timing of poison gaz by Gigas

It was too difficult to avoid it. Now, it can be avoided except if we are
going thru the south, but if the delay is too high other directions seems
odd so...
Update license, enhance poison gaz
Merge branch 'develop'