switch to mitchellh goamz, with my changes
Squashed 'vendor/go/src/github.com/mitchellh/goamz/' content from commit 590135a

git-subtree-dir: vendor/go/src/github.com/mitchellh/goamz
git-subtree-split: 590135a162c4e880d05504c03cf00261e7592f4c
Merge commit '566a46523001485636b2857ee3805f688e3e1309' as 'vendor/go/src/github.com/mitchellh/goamz'