Added new illustrations

@mglukhovsky 

* Map Reduce
    * http://192.168.0.159:8888/docs/map-reduce
* Secondary Indexes
    * http://192.168.0.159:8888/docs/secondary-indexes/ruby
    * http://192.168.0.159:8888/docs/secondary-indexes/python
    * http://192.168.0.159:8888/docs/secondary-indexes/javascript
* Data Modeling
    * http://192.168.0.159:8888/docs/data-modeling
* Table Joins
    * http://192.168.0.159:8888/docs/table-joins