Add game indices for natures.

I'm assuming (and I'm 99% sure I'm right) that the internal order for
natures has never changed.

pokedex/data/csv/natures.csv

@@ -1,26 +1,26 @@
-id,identifier,decreased_stat_id,increased_stat_id,hates_flavor_id,likes_flavor_id
-1,hardy,2,2,1,1
-2,bold,2,3,1,5
-3,modest,2,4,1,2
-4,calm,2,5,1,4
-5,timid,2,6,1,3
-6,lonely,3,2,5,1
-7,docile,3,3,5,5
-8,mild,3,4,5,2
-9,gentle,3,5,5,4
-10,hasty,3,6,5,3
-11,adamant,4,2,2,1
-12,impish,4,3,2,5
-13,bashful,4,4,2,2
-14,careful,4,5,2,4
-15,rash,5,4,4,2
-16,jolly,4,6,2,3
-17,naughty,5,2,4,1
-18,lax,5,3,4,5
-19,quirky,5,5,4,4
-20,naive,5,6,4,3
-21,brave,6,2,3,1
-22,relaxed,6,3,3,5
-23,quiet,6,4,3,2
-24,sassy,6,5,3,4
-25,serious,6,6,3,3
+id,identifier,decreased_stat_id,increased_stat_id,hates_flavor_id,likes_flavor_id,game_index
+1,hardy,2,2,1,1,1
+2,bold,2,3,1,5,6
+3,modest,2,4,1,2,16
+4,calm,2,5,1,4,21
+5,timid,2,6,1,3,11
+6,lonely,3,2,5,1,2
+7,docile,3,3,5,5,7
+8,mild,3,4,5,2,17
+9,gentle,3,5,5,4,22
+10,hasty,3,6,5,3,12
+11,adamant,4,2,2,1,4
+12,impish,4,3,2,5,9
+13,bashful,4,4,2,2,19
+14,careful,4,5,2,4,24
+15,rash,5,4,4,2,20
+16,jolly,4,6,2,3,14
+17,naughty,5,2,4,1,5
+18,lax,5,3,4,5,10
+19,quirky,5,5,4,4,25
+20,naive,5,6,4,3,15
+21,brave,6,2,3,1,3
+22,relaxed,6,3,3,5,8
+23,quiet,6,4,3,2,18
+24,sassy,6,5,3,4,23
+25,serious,6,6,3,3,13

pokedex/db/tables.py

@@ -1392,6 +1392,8 @@ class Nature(TableBase):
         info=dict(description=u"ID of the Berry flavor the Pokémon hates (if likes_flavor_id is the same, the effects cancel out)"))
     likes_flavor_id = Column(Integer, ForeignKey('contest_types.id'), nullable=False,
         info=dict(description=u"ID of the Berry flavor the Pokémon likes (if hates_flavor_id is the same, the effects cancel out)"))
+    game_index = Column(Integer, unique=True, nullable=False,
+        info=dict(description=u"This nature's internal ID in the games."))
 
     @property
     def is_neutral(self):