create mongodb delete step

Step to provide MongoDB delete functionality for Pentaho Data Integration.

![2014-06-09_115738](https://cloud.githubusercontent.com/assets/1782263/3213359/7f5e429e-ef8a-11e3-959d-0f7569111daf.png)

Build delete query base on available data from previous steps.
![2014-06-09_115847](https://cloud.githubusercontent.com/assets/1782263/3213365/ad747bf8-ef8a-11e3-939f-477c4d82f720.png)

Preview delete query:
![2014-06-09_115909](https://cloud.githubusercontent.com/assets/1782263/3213368/cc92ef42-ef8a-11e3-82cd-b82f99406ba2.png)