Support for rendering Mongo bson files

We are working with BSON data dumped from Mongo databases into HDFS. This pull request adds support for quick previewing a bson file in HDFS via Azkaban.

Summary of changes:

1. Added BsonFileViewer which encapsulates the logic of rendering a bson file.

2. Added a dependency on mongo-java-driver in lib/. This dependency is analogous to the avro dependency in hdfsviewer since it contains code to render bytes into BSON and back.

3. Changed HdfsBrowserServlet to flush and close the output stream.

4. Updated version to 2.1.1

Testing Done:

Tested against our azkaban installation.