Implement suggestion from Damir Buskulic:

- Take into account the decimal part of the time offset.
- New parameter "option" in SetTimeOffset. Default value is "local"
  meaning the time offset should be consider as a local time. It can
  be set to "gmt" meaning the time offset should be consider as GMT time.


git-svn-id: http://root.cern.ch/svn/root/trunk@7654 27541ba8-7e3a-0410-8455-c3a389f83636