Add support for -force flag on builds [GH-119]
Add instructions for how to get Go set up to contribute.
Merge pull request #173 from jasonberanek/force-build

packer, builder/*: Add support for -force flag on builds [GH-119]
Update CHANGELOG
fmt
Merge pull request #176 from smerrill/better-contributor-notes

Add instructions on how to get Go set up to contribute.
Remove output directory in a step when "-force" flag step [GH-178]
Merge pull request #179 from jasonberanek/force-build

builder/virtualbox, vmware: Remove output directory in a step when "-force" flag step [GH-178]
buidler/virtualbox,vmware: only stat if not force

/cc @jasonberanek - Swapped this because why even bother doing a stat
if we're forcing anyways.