Allows access to entities on user object

With this change, you can access the entities returned in the response for a user. 

Without this change:
```
user = Twitter.user('zanzaf')
user.entities
NoMethodError: undefined method `entities' for #<Twitter::User:0x007f95a3d65278>
```
With this change
```
user = Twitter.user('zanzaf')
user.entities
 => {:description=>{:urls=>[]}}
```