Add support for platforms

Hi there,

First of all thanks for the good job with the API wrapper. It was definitely useful for me.

I added support for platform objects, although it was a bit tricky to add the `name` attribute, because it's handled differently in lists and individual platforms:
* http://thegamesdb.net/api/GetPlatformsList.php 
* http://thegamesdb.net/api/GetPlatform.php?id=15
* https://github.com/jassa/the_games_db/blob/5eebf07a2471de7d1d28af2334a56ae9536c197b/lib/the_games_db/platform.rb#L41-62

I don't like depending on a [dynamic method from sax-machine](https://github.com/pauldix/sax-machine/blob/master/lib/sax-machine/sax_document.rb#L87-L89), but it seems to me like it's the main entry point of that data. Any suggestions?