Add instructions on how to get Go set up to contribute.

packer, builder/*: Add support for -force flag on builds [GH-119]

builder/amazonebs: Ability to specify a pre-existing SecurityGroupId for the AmazonEBS Builder

builder/digitalocean: use detected env variables for credentials

builder/amazonebs: Validate that instance exists and is EBS based before trying to launch it

builder/amazonebs: Add AWS instance id to normal output

The AWS instance id is only output when logging is occurring. If an
error happens it is useful to know the AWS instance id of the instance
spun up by packer in case further cleanup or forensics are needed.

Fix typo in packer.go

website: Just fixing a small error in the text.

The sentence was missing a few words.
I've made it read similar to the previous 3 sentences.