Add support for Ruby plugin.

This commit adds a new builder, `ruby`, which plugs into the [Jenkins Ruby Plugin](https://wiki.jenkins-ci.org/display/JENKINS/Ruby+Plugin). You can write a project that looks like: 
```
 - job:
    name: test_container
    builders:
      - ruby: |
              load(File.join(Dir.pwd, "build", "build"))
              Build.new.main
```
and get a project that looks like:
![image](https://f.cloud.github.com/assets/903181/1431810/237e6706-40d5-11e3-817a-f990998c4fd2.png)