Tin: Tune mapgen, bugfix, fix texture credits

Previously i had matched ore density to moreores, but this density was
too high and out of balance with MTGame, reduce density to be similar
to copper ore.
The 2 levels of underground ore were overlapping, fix.
Textures are actually by kilbith, update credits.

Also credit kilbith for bronze, copper, gold and steel blocks, credits
had not been updated at the time.

mods/default/README.txt

@@ -53,10 +53,6 @@ Calinou (CC BY-SA 3.0):
   default_papyrus.png
   default_mineral_copper.png
   default_glass_detail.png
-  default_mineral_tin.png
-  default_tin_block.png
-  default_tin_ingot.png
-  default_tin_lump.png
 
 MirceaKitsune (CC BY-SA 3.0):
   character.x
@@ -82,10 +78,6 @@ Splizard (CC BY-SA 3.0):
 
 Zeg9 (CC BY-SA 3.0):
   default_coal_block.png
-  default_steel_block.png
-  default_copper_block.png
-  default_bronze_block.png
-  default_gold_block.png
 
 paramat (CC BY-SA 3.0):
   wieldhand.png -- Copied from character.png by Jordach (CC BY-SA 3.0)
@@ -216,6 +208,16 @@ kaeza (CC-BY-SA 3.0):
   default_desert_sandstone_brick.png
   default_desert_sandstone_block.png
 
+kilbith (CC BY-SA 3.0):
+  default_steel_block.png
+  default_copper_block.png
+  default_bronze_block.png
+  default_gold_block.png
+  default_tin_block.png
+  default_mineral_tin.png
+  default_tin_ingot.png
+  default_tin_lump.png
+
 Glass breaking sounds (CC BY 3.0):
   1: http://www.freesound.org/people/cmusounddesign/sounds/71947/
   2: http://www.freesound.org/people/Tomlija/sounds/97669/