[phobos] std.gregorian

[Walter Bright]

Although I do not believe that SHOO's work on the date/time is legally 
infringing on Tango's time code, I feel there's been enough bad feeling 
about this and that we should not include Tango's time api design in Phobos.

I apologize to SHOO for this. I know this is unfair to him.

Andrei has given a start to std.gregorian, perhaps SHOO's implementation 
work can be transferred to this to help complete it?