[SPARK-7553] [STREAMING] Added methods to maintain a singleton StreamingContext

In a REPL/notebook environment, its very easy to lose a reference to a StreamingContext by overriding the variable name. So if you happen to execute the following commands
```
val ssc = new StreamingContext(...) // cmd 1
ssc.start() // cmd 2
...
val ssc = new StreamingContext(...) // accidentally run cmd 1 again
```
The value of ssc will be overwritten. Now you can neither start the new context (as only one context can be started), nor stop the previous context (as the reference is lost).
Hence its best to maintain a singleton reference to the active context, so that we never loose reference for the active context.
Since this problem occurs useful in REPL environments, its best to add this as an Experimental support in the Scala API only so that it can be used in Scala REPLs and notebooks.

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

Closes #6070 from tdas/SPARK-7553 and squashes the following commits:

731c9a1 [Tathagata Das] Fixed style
a797171 [Tathagata Das] Added more unit tests
19fc70b [Tathagata Das] Added :: Experimental :: in docs
64706c9 [Tathagata Das] Fixed test
634db5d [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into SPARK-7553
3884a25 [Tathagata Das] Fixing test bug
d37a846 [Tathagata Das] Added getActive and getActiveOrCreate

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