Substitute People for Person

In the [documentation of TMDB](http://docs.themoviedb.apiary.io/#people) we can see People in the list of sections and after that every endpoint is based on person, like `person/3` or `person/popular`. I have thought that a change in this gem to reflect that could be positive. 

Besides that, the methods 'popular' and 'latest' were not working before this change because they were using the paths `people/popular` and `people/latest`, this commit fixes that too.

I have also added new specs for this new entity.

Related: https://github.com/ahmetabdi/themoviedb/pull/19#issuecomment-32043724