Adding mangas

Hi, 

In this pull request I'm adding the mangas fetching.  I saw that you only fetch animes so... I've implemented the manga fetching.

I tried to not change any previous behavior. The only change I've made that modifies the method's interface is the fetch_titles, that were receiving default parameters and now is a hash, which makes possible change the request passing just what you really want to change without having to add unnecessary parameters.

All tests are passing and I implemented new ones to cover the manga fetching.

Please, feel free to review and make any comments to improve the code.