1.9 changes to EntitySelector's gamemode field

[fenhl]

In 1.9 (implemented in Notchian as of 16w02a), entity selectors will support negation of the gamemode argument, e.g. @a[m=!3] to select everyone except spectators. The syntax will also change to allow things like @p[m=c] or @p[m=creative] to select the nearest player in Creative mode, where previously only @p[m=1] was allowed. To implement this we need to:

• [ ] make the gamemode field on types::selector::EntitySelector an Attr<u8> instead of Option<u8>
• [ ] change the FromStr impl to parse the field correctly
• [ ] change the From<&EntitySelector> for String impl to write gamemode using push_attrs! instead of push_args!

This would also be a great opportunity to introduce a gamemode enum and use it everywhere, including selectors, the clientbound packets JoinGame and Respawn, and the server.properties parser.