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.
Merge pull request #34 from nxtelevision/30-update-modified-on-nxman-conversion

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