Add is_legendary and is_mythical (#304)

Legendary and Mythical are official terms which, while generally not
present in the game data, are more or less consistently applied in
promotional material. They're also useful categorizations to fans,
so it seems reasonable to include them here.

The legendary Pokémon are:
  Articuno, Zapdos, Moltres, Mewtwo,
  Raikou, Entei, Suicune, Lugia, Ho-Oh,
  Regirock, Regice, Registeel,
  Latias, Latios, Kyogre, Groudon, Rayquaza,
  Uxie, Mesprit, Azelf, Dialga, Palkia, Giratina,
  Heatran, Regigigas, Cresselia,
  Cobalion, Terrakion, Virizion,
  Tornadus, Thundurus, Landorus,
  Reshiram, Zekrom, Kyurem,
  Xerneas, Yveltal, Zygarde,
  Tapu Koko, Tapu Lele, Tapu Bulu, Tapu Fini,
  Cosmog, Cosmoem, Solgaleo, Lunala, Necrozma.

The mythical Pokémon are: Mew, Celebi, Jirachi, Deoxys, Phione, Manaphy,
  Darkrai, Shaymin, Arceus, Victini, Keldeo, Meloetta, Genesect,
  Diancie, Hoopa, Volcanion, Magearna, Marshadow, Zeraora.

pokedex/db/tables.py

@@ -2055,6 +2055,10 @@ class PokemonSpecies(TableBase):
         doc=u"ID of the growth rate for this family")
     forms_switchable = Column(Boolean, nullable=False,
         doc=u"True iff a particular individual of this species can switch between its different forms.")
+    is_legendary = Column(Boolean, nullable=False,
+        doc=u'True iff the Pokémon is a legendary Pokémon.')
+    is_mythical = Column(Boolean, nullable=False,
+        doc=u'True iff the Pokémon is a mythical Pokémon.')
     order = Column(Integer, nullable=False, index=True,
         doc=u'The order in which species should be sorted.  Based on National Dex order, except families are grouped together and sorted by stage.')
     conquest_order = Column(Integer, nullable=True, index=True,