Add plugin class

This patch adds plugin class to provide API.
User can get plugin related status using the below url.

  $JENKINS_URL/plugin/rabbitmq-consumre/api/xml

To get full status, you need to add "depth=1" parameter to this URL.

Expected status format is:

```xml
<rabbitMQConsumer>
  <connected>true</connected>
  <enabled>true</enabled>
  <queue>
    <appId>APPID</appId>
    <consumed>true</consumed>
    <name>QUEUENAME</name>
  </queue>
  <serviceUri>amqp://localhost</serviceUri>
  <userName>USERNAME</userName>
</rabbitMQConsumer>
```