Add the ability to restrict publish plugins to a specific branch

This pull request adds the ability to restrict running publish plugins to a specific branch.  This functionality uses the unused `branch` option that the s3 and swift plugins had already defined.  Additionally the option was extended to the PyPI plugin.

An example would be only wanting to publish a file after a successful build/test to the `master` branch.  Existing functionality remains unchanged, running publish plugins on all branches if `branch` is not specified.

An example would look like:

```
publish:
  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
    branch: master
```

In addition to this change, since `*repo.Repo` is passed down to `publish.Publish.Write` publish plugins in the future could utilize all information provided by the Repo struct by passing it in to the individual plugins `Write` method.

This could also be extended to deployment plugins, etc...I don't have a use case at the moment outside of publish plugins, so for now I've restricted it to that.