Allow using CocoaPods to install the library

* Moved DemoViewController.{h,mm} from the main folder to the demo folder. 

* Create a .podspec file and tested it. Example of usage: 

======== Example Podfile ======== 
platform :ios, '5.0'

pod 'ZBarSDK'
pod "QR-Code-Encoder-for-Objective-C", :git => "https://github.com/jasperblues/QR-Code-Encoder-for-Objective-C.git"


* I would like to push the .podspec to the main CocoaPods spec repo. . This allows using the short-hand style, like with ZBarSDK above. . . before I do this:  It would be better to have a tag to refer to as the version.