Update "modified" attribute on NXman conversion (resolves #30)

When converting a User object to a NXman object, using the class method "Nxman::createFromUser()", the "modified" attribute is now updated to the current date/time.
Solves #30.

This commit shouldn't break anything since it is very trivial. Can you please take a look at it and merge it right away if you don't mind, @Zboubidoo ?