[SPARK-6796][Streaming][WebUI] Add "Active Batches" and "Completed Batches" lists to StreamingPage

This PR adds two lists, `Active Batches` and `Completed Batches`. Here is the screenshot:

![batch_list](https://cloud.githubusercontent.com/assets/1000778/7060458/d8898572-deb3-11e4-938b-6f8602c71a9f.png)

Due to [SPARK-6766](https://issues.apache.org/jira/browse/SPARK-6766), I need to merge #5414 in my local machine to get the above screenshot.

Author: zsxwing <zsxwing@gmail.com>

Closes #5434 from zsxwing/SPARK-6796 and squashes the following commits:

be50fc6 [zsxwing] Fix the code style
51b792e [zsxwing] Fix the unit test
6f3078e [zsxwing] Make 'startTime' readable
f40e0a9 [zsxwing] Merge branch 'master' into SPARK-6796
2525336 [zsxwing] Rename 'Processed batches' and 'Waiting batches' and also add links
a69c091 [zsxwing] Show the number of total completed batches too
a12ad7b [zsxwing] Change 'records' to 'events' in the UI
86b5e7f [zsxwing] Make BatchTableBase abstract
b248787 [zsxwing] Add tests to verify the new tables
d18ab7d [zsxwing] Fix the code style
6ceffb3 [zsxwing] Add "Active Batches" and "Completed Batches" lists to StreamingPage