Support GPG credentials, align bucket spec with lein repository semantic...

I added support for GPG credentials.  I think  it's best to have the specification of the amazon bucket and credentials work the same way that lein specifies maven repos/credentials, and just use the associated lein code to parse the credentials. This requires two changes to :deploy-app in project.clj for users of the plugin:

1. Use the key :url instead of :s3-bucket (value remains the same).
2. Instead of {:cred env} use something like {:username :env :passphrase :env}.

So, for example  
```clojure
(defproject my-project "0.1.0"
  ... 
 :deploy-app {:url "s3p://MY-BUCKET/releases" :creds :gpg}
  ...)
```
or:
```clojure
(defproject my-project "0.1.0"
 ... 
 :deploy-app {:url "s3p://MY-BUCKET/releases" :username :env :passphrase :env}
 ...)
```

Or otherwise as specified in the [lein deploy docs](https://github.com/technomancy/leiningen/blob/master/doc/DEPLOY.md#authentication).