READY: Changes to enable multi-package builds.

If you have a "packages" key containing a list in your ploy-publisher.yml, ploy will now loop through the list, merge the main configuration with any keys you specify, and then build a package for each item in the list.

E.g.:

```
---
bucket: mantacore-deployment
dist_dir: .
prefix: /usr/local/deployment-test-repo
upstart_files:
  - init/deployment-test-repo
packages:
  - deploy_name: deployment-test-repo-a
    postinst: echo foo
  - deploy_name: deployment-test-repo-b
    postinst: echo bar
```

So, the dist_dir, prefix, bucket, etc. will be inherited by -a and -b.

cc/ @khadrin