builders/googlecompute: fixed link in docs.

Clarify error message in case of an atlas client connection failure

When a packer json contains an atlas postprocessor, and `packer validate` runned locally
in case of the missing ATLAS_TOKEN env var, the following error message is displayed:

* Error initializing client: authentication failed

It wasn't clear which plugin produced this message, so the amazon-ebs builder was the suspect

Add Atlas post-processor documentation

Update Push APIs

This allows children to get the raw templates without re-reading the
file.

Fixes #1881 Don't include Windows Disks when packaging boxes for prl

This excludes the directory "Windows Disks" present in the VM data
directory if you hava a Windows VM with Parallels tools installed.

Add Travis CI badge to README
[skip ci]

Fix page title
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Use the new Travis Docker infrastructure and stuff

Fix tutorial source ID
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`ami-9eaa1cf6` is the correct Ubuntu 14.04 AMI for the us-east-1 region.

Fixes #1841

Adding documentation for the environments_path
[skip ci]

Docs fixes for shell provisioner page

website/intro/getting-started: clarify that multiple builders of the same type must have a unique name

Fixes #1718 
[skip ci]

[skip ci]