Embedding spark execution engine.

This pull request is about how we can integrate spark into zeppelin. to do ZEPPELIN-66.

What i did in this pull request is create an spark context and pass it to Zeppelin Library.
Also introduced a concept preprocess() postprocess() in Zeppelin Library.
So any zeppelin library can implement preprocessing() postprocessing() with Scala code which can leverage spark context.

However, this pull request will never be merged. 
Because it depends on spark to much. too tightly integrated. i'm thinking less tight integration.

this pull request is just for reading. fun reading.