[SPARK-7530] [STREAMING] Added StreamingContext.getState() to expose the current state of the context

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #6058 from tdas/SPARK-7530 and squashes the following commits:

80ee0e6 [Tathagata Das] STARTED --> ACTIVE
3da6547 [Tathagata Das] Added synchronized
dd88444 [Tathagata Das] Added more docs
e1a8505 [Tathagata Das] Fixed comment length
89f9980 [Tathagata Das] Change to Java enum and added Java test
7c57351 [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into SPARK-7530
dd4e702 [Tathagata Das] Addressed comments.
3d56106 [Tathagata Das] Added Mima excludes
2b86ba1 [Tathagata Das] Added scala docs.
1722433 [Tathagata Das] Fixed style
976b094 [Tathagata Das] Added license
0585130 [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into SPARK-7530
e0f0a05 [Tathagata Das] Added getState and exposed StreamingContextState