[GH-152]

Currently the passed in AWS auth or AWS environment variables are
interpreted by packer. This change moves that logic into goamz in
order to support both the existing and instance based IAM role
authentication. This requires a corresponding change to goamz.

/cc @markpeek I think this is much cleaner. Also, I made it so it
doesn't append by default... I like the idea of appending but it scares
me that for a bug report someone might put like a multi-megabyte gist of
logs at me. HAHA. We'll see.

Add ability to send log output to a file using PACKER_LOG_PATH

When the environment variables PACKER_LOG and PACKER_LOG_PATH are both
set the log output will be appended to the PACKER_LOG_PATH file.

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

builder/virtualbox, vmware: Remove output directory in a step when "-force" flag step [GH-178]

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