Renamed GDC repository

[Iain Buclaw]

FYI - new name / location of repository is: 
https://github.com/D-Programming-GDC/gdc

This is a change from UPPER case (GDC) to lower case (gdc), which 
may require anyone who has a local repository to update the 
remote location path before pulling. Though I guess most normal 
people will just re-clone. =)

Thanks,
Iain