Wiki updates

[Johannes Pfau]

I restructured the http://gdcproject.org/wiki/Installation page, I hope
it's OK this way.

I also added a Raspberry Pi overview page
http://gdcproject.org/wiki/Raspberry Pi and a new installation guide
for raspbian
http://gdcproject.org/wiki/Raspberry%20Pi/build/raspbian_hardfloat
would be great if someone could test the raspbian guide and report any
success or failures. It works for me, but I'm not sure if I forgot to
document some details.

@Iain the home page (http://gdcproject.org/wiki/Home) could need some
small updates:

Frontend version still says 2.059 and links to the 2.057 changelog.

There are still some references to the old issue tracker:
"A bug, proposal, or enhancement can go to the issue tracker on
bitbucket."
"This list is still maintained, however, new bugs should be submitted
to the Bitbucket repository."

And this should probably be changed to refer to git commits:
"Include the hg changeset or commit you are using for GDC. (e.g. commit
114 or changset 6f03952ff48f)"

If you give me permission to edit the home page (user JohannesPfau) I
can fix those, if not that's fine as well ;-)

BTW:
Would it be possible to add links to the gdcproject.org main page,
github repository, bug tracker and D.gnu forum to the wiki Quick links
list?