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

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.

Using PACKER_LOG=1 causes all the log output to be sent to Stderr. This
change maintains that backward compatility. Anything other than "1" will
be treated as a filename which will have logging appended to that file.
This is useful, for example, to always have debugging available without
cluttering up stdout (and without having to redirect stderr all the time).

Fix typo in packer.go