Adding the ability to specify a security_group_id for the amazonebs builder
builder/digitalocean: use detected env variables for credentials
builder/digitalocean: add tests for credentials via env vars
Merge pull request #164 from mitchellh/do-env-vars

builder/digitalocean: use detected env variables for credentials
Update CHANGELOG
Merge pull request #162 from brendanhay/persistent-security-group

builder/amazonebs: Ability to specify a pre-existing SecurityGroupId for the AmazonEBS Builder
website: add docs for new security_group_id
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.