HPCC-8572 Std.Date functionality

Time support added to Std.Date; new timelib plugin; stringlib
modifications to support time; updated tests for new Std.Date
functionality; additional date manipulation functions for
Gregorian dates.
Unfortunately %F is c99, which isn't supported by visual studio 2008.  However we don't document which versions of visual studio we support - so I think we should use %F, but we may need a note somewhere, and check the constant folding works with our build of the windows client tools.