Added Trigger class

WIP - @cowboyd thoughts on what this error means? It occurs when I visit the configuration page of a job/project.

```
javax.servlet.ServletException: org.apache.commons.jelly.JellyTagException: jar:file:/Users/drnic/Projects/ruby_jpi_plugins/build-change-trigger-plugin/work/war/WEB-INF/lib/jenkins-core-1.502.jar!/lib/hudson/project/config-trigger.jelly:31:84: <j:invokeStatic> method for_ threw exception: org.jruby.proxy.hudson.model.Descriptor$Proxy1 cannot be cast to hudson.triggers.TriggerDescriptor
	at org.kohsuke.stapler.jelly.JellyFacet$1.dispatch(JellyFacet.java:103)
	at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:573)
	at org.kohsuke.stapler.Stapler.invoke(Stapler.java:658)
```

I have a subclass like this:

``` ruby
java_import Java.java.util.logging.Logger

class JobUpdatedTrigger < Jenkins::Triggers::Trigger

  display_name "Job updated trigger"

  attr_accessor :watching_job_name

  def logger
    @logger ||= Logger.getLogger(self.class.name)
  end

  # Invoked with the form parameters when this extension point
  # is created from a configuration screen.
  def initialize(attrs = {})
    @watching_job_name = attrs['watching_job_name']
  end

  def run
    logger.info "JobUpdatedTrigger#run"
  end

  def stop
    logger.info "JobUpdatedTrigger#stop"
  end
end
```

And a descriptor config.erb like this:

``` ruby
<%
f = taglib("/lib/form")
f.entry(:title => 'Job name to watch', :field => 'watching_job_name', :description => "Triggers based on any updates to job configuration") do
  f.textbox
end
%>
```