Adds tags method to Giphy::Gif object that returns tags.

We add a method to be able to access the tags from a Giphy::Gif object. Previously, there was no method to access the "tags" in the hash. 

For example, 

> gif = Giphy.random
 => #<Giphy::Gif:0x007fd962e1bd68 @hash={"id"=>"eanEvrelAMabe", "image_original_url"=>"http://media.giphy.com/media/eanEvrelAMabe/giphy.gif", "image_url"=>"http://media.giphy.com/media/eanEvrelAMabe/giphy.gif", "tags"=>["tv", "assassins creed brotherhood"]}> 
> gif.tags
=> ["tv","assassins creed brotherhood"]