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