Image Manager reference counting.

* Make the image manager the defacto loader for
Image resources.

* Users of image resource need to maintain their
references.

* New helper class ofxGenericImageCache can maintain a set of image
references.
Moved image creation from files to image manager.

* Moved static method getNativeImagePath to ofxGenericImageManager.

* Removed ofxGeneric::create/init from file methods.

* Remade other create/inits public and removed friend interfaces.
Refactor ofxGenericImage create from file.

Refactored code to make ofxGenericImage::create() the interface for
loading images from file resources.

* Removed ofxGenericImageManager::load() interfaces, added add()

* Recreated ofxGenericImage::create(file)

* Added ofxGenericImage::createAsync(file)

* Refactored image loading in view classes.
setBackgroundImage sets UIImage to null.
Merge pull request #9 from lumoslabs/palange/fix_img_mgr

Image Manager refactoring.
reviewed with eli-lumos