Add keepOriginal attribute

keepOriginal will controll whether gimlet deletes splash source files
after copying then to the final destination. This can potentially save
huge chunks of app size.
Add keep_original attribute to image object

keep_original will be used to control whether the source file for
splash images get deleted after copy.
use global syntax for keeporiginal flag
Use language naming conventions
Merge pull request #28 from bobeast/master

Add keepOriginal attribute