Support timezone designator in timestamp string - fix Issue#58

A happy ending of this whole week of bug blitz of oBIX 1.2 release, here comes the fix to Issue#58 - no timezone support as in ISO-8601 standard.

Valgrind test, make-test, historyQuery tests all passed.

Comments please refer to commits head, test results please refer to Issue#58. In particular, note that due to the limitation of the implementation of strptime() C API, only "+/-hh" and "+/-hhmm" formats are recognised.