VM: Looking for built-in classes, named Jack_Xxx

The emulator is now looking for built-in VM classes named Jack_Xxx
and then falls back to just Xxx. This removes the naming conflicts
for classes like String.java.
Cosmetics: String now resolves to java.lang.String
Merge pull request #1 from itoshkov/built-in-vm

VM: Improved naming conventions for built-in Jack classes