Added backgroundColor Property and Podspec file

I added a `backgroundColor` property to SJNotificationViewController to change its background color regardless of its notification level. The `backgroundColor` property also adds the ability for users the give the notification different colors per view controller that SJNotificationViewController is used in. I updated the README with the documentation on the new feature.

I also added a Podspec file so that this project can be used with [CocoaPods](http://cocoapods.org) its like Bundler for Objective-C. Check out this [tutorial on CocoaPods](http://nsscreencast.com/episodes/5-cocoapods). I thought this a great project so why not add it to CocoaPods? 

Once you accept the pull request you can remove the second `s.author` ,`s.homepage`  and `s.source` lines in the podspec file.