Adding Pokemon to Sky Battle

As according to http://www.serebii.net/xy/skybattles.shtml

The following has been added to the whitelist: Azelf, Cresselia, Eelektrik, Eelektross, Koffing, Landorus, Latias, Latios, Mesprit, Misdreavus, Mismagius, Thundurus, Tornadus, Tynamo, Unown, Uxie, and Weezing.

Ho-Oh, Lugia, and Rayquaza are excluded from the whitelist as they're Ubers.

On a side note, Thundurus-T can float but probably hasn't been tested by Serebii yet. I didn't add it to the whitelist but I do anticipate it to be at some point.