GDMD D port ready for alpha-testing

[Johannes Pfau via D.gnu]

I've finally finished the D port of GDMD and pushed everything
including a detailed README to
https://github.com/D-Programming-GDC/GDMD/tree/dport

All parameters are tested for simple cases but in complex
combinations there could still be some unknown bugs.


I think the D port should already be much more useful than the perl
script though. It supports gdmd -main, gdmd -run and gdmd -lib as well
as most other options in DMD 2.066*. And it properly creates all output
folders which dmd-script sometimes doesn't do...

So I'd like to ship this gdmd with the binaries starting with the next
release. @Iain if you've got no objections I'd also like to merge this
into the GDMD master branch ASAP. We have to keep dmd-script in master
though for travis-ci.


* There are some DMD options which can't work the same way in GDC. For
  example treating deprecation as an error but all other warnings as
  warnings.