Update XnatUtils.py

Reorganizing the functions in XnatUtils and adding function to get_resource_lastdate_modified() to get the last update date for a resource on xnat and it returns a string yearmonthdayhourminutesecond (e.g: 20130928022727 meaning 09/28/2013 at 02:27:27)
Merge remote-tracking branch 'origin/master' into faster_modules