Added Web Images Support =)

-Web image support using SDWebImage (more accurately, 'UIActivityIndicator-for-SDWebImage') Pod: now the JPSThumbnail has a new property named: imageURL where the URL for the thumbnail can be set, instead of the image property (this latter one must be left as nil in order to use the imageURL property). 

-Changed demo project to Use Cocoapods, in order to be able to Use the UIActivityIndicator-for-SDWebImage Pod

-Added the UIActivityIndicator-for-SDWebImage dependency into the podspec file.

-Updated README file to reflect the new feature.