Person features

Based on the movie's features, I made some basic API calls.

Method #find that gets the basic information about a person given a specific ID.
```ruby
person = TMDb::Person.find(138)
```
Method #images that gets the images for a specific person ID.
```ruby
images = TMDb::Person.images(138)
```

Method #popular that returns a list of popular people from TMDb. Accepts page option.
```ruby
popular = TMDb::Person.popular
popular = TMDb::Person.popular(page: 4)
```