Support for S3 and Asset Library Code refactoring

Support for s3 storage in asset library
Asset Library Refactoring
	* Image and audio Processors are separated from attachment_mixin
	* image size config is separated from attachment_mixin
	* Storage is separated from attachment_mixin
	* s3 storage is introduced
	* rake task is made to migrate public folder assets to s3
	* All other asset library functionality is tested and fixed for new refactored code.