Bug 1080250 - Add support for Windows 8 x64

Treeherder equivalent of:
https://hg.mozilla.org/webtools/tbpl/rev/547a7de112ac
plus the followup bug 1085549.

(This is rebased on PR #256 to avoid future conflicts, so please look at the diff directly or wait until that PR is merged to master, so the PR is cleaner)