Core: New type of user attribute timezone.

User attribute timezone is now in standart of tz database, http://en.wikipedia.org/wiki/List_of_IANA_time_zones.
- the timezones in string are typed in the module and added to HashSet,
  since HashSet has constant time complexity of searching
- timzone attribute modul edited, so as it checks the value according to
  the timezones in set
- tests modified
Merge pull request #78 from jirmauritz/timezone

Core: New type of user attribute timezone.