Feature/document facts

Hello,

this branch is based on https://github.com/ansible/ansible/pull/3000 and it adds the latest information about ubuntu/debian and also a small section about the "setup" module.

Please let me know if it looks suitable and feel free to reject if you feel its redundant. 

My rational is that the conditional task execution is a very powerful feature and the "setup" fact gathering includes a wealth of information that is even for ad-hoc calls useful so I felt it should be given more prominent space than just mentioning it in the FAQ only (I implemented a custom is_vmware fact module before I figured out that its already provided (but I could/should have guessed that it is!) :)

Thanks for considering!
 Michael