Merge pull request #1 from gamaral/date-extension

Sieve Date Extension, Sieve Index Extension, CurrentDate Fix
sieve/ : prefer reentrant time functions

-- Replace calls to localtime() and gmtime() with calls to localtime_r()
   and gmtime_r(), so that libcyrus_sieve can be used in multithreaded
   applications.
Fix use of gmtoff_of (spotted by Дилян Палаузов)