Add OpenStack swift publish functionality

This pull request adds OpenStack Swift support as a publish method, which will also support Rackspace Cloud Files.

An example use for Rackspace Public Cloud would look like:

```
  swift:
    username: someuser
    password: 030e39a1278a18828389b194b93211aa
    auth_url: https://identity.api.rackspacecloud.com/v2.0
    region: DFW
    container: drone
    source: /tmp/drone.deb
    target: latest/drone.deb
```