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.
Part of me likes this - because it is easy to convert to seconds and intuitive etc..  
Part of me hates it because using a floating point number for something with a specific accuracy isn't really right.

I would be more inclined to use an unsigned8 time in milliseconds instead.  @richardkchapman your opinion?