WIP: replace Hpricot with Nokogiri

I've been working on replacing Hpricot with Nokogiri everywhere in the project. It's used for the oneboxes.

The main reason for this is that we're using these 2 gems that serve the same purpose, so I think we should choose one of them. As Nokogiri is more actively maintained and has benchmarks that prove it's faster, I've gone with it.

I've added specs for the oneboxes that were using Hpricot (amazon, apple, android, flickr and wikipedia), and then I've replaced the gems.

The change has been quite transparent, as Nokogiri features an Hpricot compatibility layer.

Before I send a formal pull request, I'd like to hear your opinion on this (maybe there was another reason to use Hpricot), and if everything looks OK, I'll add another commit to fake the web requests on the specs.

Thanks!