Add Vanity class and add new fields to Player.

This pull request adds 3 new things to the hypixel-ruby client:
* A field in the [player](https://github.com/Cryptkeeper/hypixel-ruby/blob/master/lib/hypixel-ruby/player.rb) class that holds the amount of Hypixel credits the player has.
* A field in the [player](https://github.com/Cryptkeeper/hypixel-ruby/blob/master/lib/hypixel-ruby/player.rb) class that holds the rank of the player.
* A class that holds a player's packages from the vanity shop.

I also added syntax highlighting to the README and fixed a small error on the `gem build` command in the installation instructions :smile: