Add how Packer fits into the HashiCorp Ecosystem

Delete GCE disk on SIGINT.

Use new Google API and OAuth libs, add UserAgent string

Address certain mobile issues

Explain how packer works with atlas

Clarify the plugin discovery search & priority orders.

Corrected invalid boot command section for Parallels, Virtualbox, and VMware ISO builders

Better error reporting when a config key in template is Unknown

This patch will allow to fix the following bug much faster:

```
1 error(s) occurred:

* Unknown configuration key: output_directory
```

Related configuration:

```
"output_directory ": "build/sl_base/",
```

After the patch, the error reporting will be:

```
1 error(s) occurred:

* Unknown configuration key: "output_directory¤"
```

Address go vet reports

Fixes the following vet reports:

provisioner/salt-masterless/provisioner.go:119: arg cmd for printf verb %s of wrong type: *github.com/mitchellh/packer/packer.RemoteCmd
provisioner/salt-masterless/provisioner.go:121: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:142: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:157: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:172: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:216: arg err for printf verb %d of wrong type: error

Fixes the following vet report:

provisioner/puppet-masterless/provisioner.go:196: missing argument for Errorf(%d): format reads arg 1, have only 0 args

Fixes the following vet reports:

post-processor/vagrant-cloud/client.go:81: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:99: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:131: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:156: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:175: arg resp for printf verb %s of wrong type: *net/http.Response

Fixes the following vet report:

post-processor/vagrant/post-processor_test.go:135: possible formatting directive in Fatal call

Fixes the following vet reports:

packer/template_test.go:496: possible formatting directive in Fatal call
packer/template_test.go:528: possible formatting directive in Fatal call

Fixes the following vet report:

communicator/ssh/communicator_test.go:88: range variable newChannel captured by func literal

Fixes the following vet reports:

common/step_create_floppy_test.go:79: possible formatting directive in Fatal call
common/step_create_floppy_test.go:89: possible formatting directive in Fatal call
common/step_create_floppy_test.go:180: possible formatting directive in Fatal call
common/step_create_floppy_test.go:190: possible formatting directive in Fatal call

Fixes the following vet report:

builder/vmware/iso/builder_test.go:178: arg b.config.DiskSize for printf verb %s of wrong type: uint

Fixes the following vet report:

builder/vmware/common/step_shutdown_test.go:130: missing argument for Fatalf("%s"): format reads arg 1, have only 0 args