[SPARK-7532] [STREAMING] StreamingContext.start() made to logWarning and not throw exception

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

Closes #6060 from tdas/SPARK-7532 and squashes the following commits:

6fe2e83 [Tathagata Das] Update docs
7dadfc3 [Tathagata Das] Fixed bug again
99c7678 [Tathagata Das] Added logInfo
65aec20 [Tathagata Das] Fix bug
5bf031b [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into SPARK-7532
1a9a818 [Tathagata Das] Fix scaladoc
c584313 [Tathagata Das] StreamingContext.start() made to logWarning and not throw exception