Add plugin for the Redmine issue tracker REST API.

This implements a client for the Redmine REST API, described here: http://www.redmine.org/projects/redmine/wiki/Rest_api

An additional API key configuration is required, like:

```json
  "api_keys": {
    "redmine": {
      "url": "https://redmine.endpoint.com/issues/",
      "key": "6b43ab43c4eeff1bdeafd3610a898ce94213702b"
    }
  },
```

We're using this in production.  We may add the ability to post updates later.