Allow calls to data in languages other than 'english'

The actual code allows not to grab data in languages other than 'english', even this being a feature of TMDB v3 API.

This allows an optional parameter `language` (defaults to `en`) to .configure() globally, and allows per-call override of it if needed. An use case is when one try some data that is not available in you local language, than he can try again in english.

This use-case is described at: http://docs.themoviedb.apiary.io/ , Languages session. There is some guidance about the feature at http://help.themoviedb.org/discussions/problems/1061-v3-api-search-in-local-language too.