Document shell provisioner example for FreeBSD

Detect the first public IPv4 address for Digital Ocean

builder/googlecompute: default SSH settings properly [GH-2340]

builder/amazon/instance: set valid bundle prefix [GH-2328]

builder/amazon: output WinRM password for debug mode [GH-2336]

packer: validate minimum version [GH-2310]

Fix flags so we can do e.g. -only=build1,build2

AppVeyor: only build master branch

AppVeyor builds all branches per default.
Add a configuration setting to only build the master branch.

See http://www.appveyor.com/docs/branches#white-and-blacklisting for details.

For the record, AppVeyor builds can skipped by annotating the commit message
according to http://www.appveyor.com/docs/how-to/skip-build
Useful for e.g. website commits.

[skip appveyor]

Add AppVeyor status badge to README

Add AppVeyor configuration file

Add knife config by template

website: Update docs for default configuration template

builder/vmware: Run stat on vmware when checksum type = none

Fix setting ami_groups

Fixes spot instance cleanup.

website: Update some docs to point to Atlas instead of Vagrant Cloud

website: Tips for debugging a build.

#2346 updated to AMI that doesn't require accepting marketplace terms

builder/aws: fix adding launch permissions for created ami.

Packer lacks an official Windows CI server.
This commit adds a configuration file for AppVeyor, which is a Windows CI
service (free for open-source projects).

Updated to same ami as in main readme.md which is from community AMIs
and therefore doesn't require accepting the AWS marketplace terms
before using in Packer build.