Added DataStoreOutputOperator, datastore writer interface and mongodb im...

...plementation for batch inserts

Squashed commit of the following:

commit 2c4062f8582cd1736c9d3f41a2414a6c9bbf2675
Author: Ashwin Chandra Putta <ashwin@datatorrent.com>
Date:   Tue Mar 25 20:11:17 2014 -0700

    cleanup and documentation

commit a6974db5ecd4bd1b611c20829f447005f6c2283e
Merge: 7c1de04 a71f615
Author: Ashwin Chandra Putta <ashwin@datatorrent.com>
Date:   Tue Mar 25 17:50:23 2014 -0700

    Merge branch 'ETL' of https://github.com/DataTorrent/Malhar into etlout

commit 7c1de04e104056730deb53f0a5c1cc00fabf6afe
Author: Ashwin Chandra Putta <ashwin@datatorrent.com>
Date:   Mon Mar 24 18:41:57 2014 -0700

    added datastore interface which can be used for implementing writes and reads from databases