Move obix_get_timestamp from client library to common library

Rename obix_get_timestamp() to get_utc_timestamp() since it invokes
gmtime_r() to convert calendar time into UTC based struct tm, and
move it from the obix-client library to obix-common library so as
to be used by test tools without having to link up with obix-client
library which requires libcurl as well.
Move the inclusion of XML headers from obix_utils.h to xml_utils.h

This frees obix_utils.h from XML headers so that test tools can
manipulate libobix-common without having to link up with libxml2.
Add a new test case to convert timestamp string into a new one
that is in GMT timezone.

If no timezone designator available in the string, or "Z" is
used as the last byte, the timestamp will be considered in UTC
timezone already.

Otherwise it will apply the timezone designator and make a new
timestamp string in UTC timezone.

1.
[harry@ndclin004 tools]$ ./ts2utc 2014-10-7T16:38:24
Input timestamp: 2014-10-7T16:38:24, remnant: (null)
Calender time in GMT timezone: 1412699904
New timestampe: 2014-10-07T16:38:24

[harry@ndclin004 tools]$ ./ts2utc 2014-10-7T16:38:24Z
Input timestamp: 2014-10-7T16:38:24Z, remnant: (null)
Calender time in GMT timezone: 1412699904
New timestampe: 2014-10-07T16:38:24

2.
[harry@ndclin004 tools]$ ./ts2utc 2014-10-7T16:38:24-10:00
Input timestamp: 2014-10-7T16:38:24-10:00, remnant: :00
Calender time in GMT timezone: 1412735904
New timestampe: 2014-10-08T02:38:24

[harry@ndclin004 tools]$ ./ts2utc 2014-10-7T16:38:24-1000
Input timestamp: 2014-10-7T16:38:24-1000, remnant:
Calender time in GMT timezone: 1412735904
New timestampe: 2014-10-08T02:38:24

[harry@ndclin004 tools]$ ./ts2utc 2014-10-7T16:38:24+1000
Input timestamp: 2014-10-7T16:38:24+1000, remnant:
Calender time in GMT timezone: 1412663904
New timestampe: 2014-10-07T06:38:24

[harry@ndclin004 tools]$ ./ts2utc 2014-10-7T16:38:24+10
Input timestamp: 2014-10-7T16:38:24+10, remnant:
Calender time in GMT timezone: 1412663904
New timestampe: 2014-10-07T06:38:24

3.
[harry@ndclin004 tools]$ ./ts2utc 2014-2-28T16:38:24-10
Input timestamp: 2014-2-28T16:38:24-10, remnant:
Calender time in GMT timezone: 1393641504
New timestampe: 2014-03-01T02:38:24

[harry@ndclin004 tools]$ ./ts2utc 2012-2-28T16:38:24-10
Input timestamp: 2012-2-28T16:38:24-10, remnant:
Calender time in GMT timezone: 1330483104
New timestampe: 2012-02-29T02:38:24

[harry@ndclin004 tools]$
Support ISO-8601 timestamp format