Adds json api for stages, storage and executor pages

This is an early PR to discuss about the usefulness/interest in community towards having json api for spark application ui.

We have found following uses with the api:
1. Build query centric ui - combine stages of same job for easy understanding and analysis.
2. Automated tools could use the apis to alarm on stages fail or cache memory full etc.,
3. Generate timely reports on total stages run, least and highest response times etc.,

Would be happy to make any style changes, add other end points and re-send a pull request on the master branch.