xREL.v3

Fix Traveller achievement. (Jim's fault! Sorry.)
Make all zones/weapons/missions achievements unlock when loading save games.
Make rank achievements unlock when attaining any higher rank (so, e.g., hitting rank 3 will also grant the rank 1 achievement if you don't already have it).
Reverted to non-exclusive fullscreen. We tried this out because a few people had requested it and exclusive was slightly faster, but it proved too annoying. Unfortunately because it's a build-only function in Unity we can't offer it as a game option.
Added difficulty selection. This is a bit of a work in progress as it only currently changes the damage taken by the player. In future we might also look at imposing different behaviour "personalities" on these guys depending on difficulty options. Beware Easy is VERY EASY Difficulty settings can be applied in your current game using the escape menu.
Added tourist camera option (see readme.txt in game folder root).
Included lorebook PDF in game folder for ytour convenience - this is still in the manual for the Steam page, too.
Fixed some scoped weapons inverting axes when zoomed.
Dropguns now take damage on their wings.
Maze achievement timer no longer keeps counting if you transfer to a new drone from the maze.
Increased size/legibility of contextual help.
Hiding HUD no longer changes overall game colour.
Added readme file to explain tourist mode.