Updates to person.rb

When using the gem in my project I had issues with `person.credits` not displaying all credits, only movie credits. I updated the gem to include methods for `person.credits => Tmdb::Person.credits` which now queries `combined_credits`. If the credits still need to be split between movie and TV, I included `person.tv_credits => Tmdb::Person.tv_credits` which queries `/tv_credits` and `person.movie_credits => Tmdb::Person.movie_credits` which queries `/movie_credits`. While I was at it, I decided to include the other missing methods for People including:
`person.external_ids => Tmdb::Person.external_ids` which queries `/external_ids`
`person.tagged_images => Tmdb::Person.tagged_images` which queries `/tagged_images`