Local mode for VBoxGuestAdditions. Provide local path and SHA256

Solves: https://github.com/mitchellh/packer/issues/131

Both VBoxGuestAdditions path and SHA256 checksum are optional.

If local/remote path is not provided, get it from virtualbox.org
If SHA256 checksum is not provided, get it from virtualbox.org

Remote url:
```
"builders": [
  {
    "type": "virtualbox",
    "guest_additions_url": "http://download.virtualbox.org/virtualbox/4.2.16/VBoxGuestAdditions_4.2.16.iso",
    "guest_additions_sha256": "b9afd356d85fe6822fd28009fd63faddd2ac7cbd75c415ea4ce6a22925243ed3"
  }
]
```

Local path:
```
"builders": [
  {
    "type": "virtualbox",
    "guest_additions_url": "/path/to/VBoxGuestAdditions_4.2.16.iso",
    "guest_additions_sha256": "b9afd356d85fe6822fd28009fd63faddd2ac7cbd75c415ea4ce6a22925243ed3"
  }
]
```