Grade Refactoring and code cleaning

Added to the Grade DataItem view the code for the scale
grades management. Now is posible to use letter scales or any
other scale.
Order modules by types inside the section
Scorm package download and extraction draft

This works on Android at the moment.
Some files have been renamed and the Android ExtractZipFilePlugin
has been fixed. It didn't work with archives containing folders.
Now it works.