[SPARK-1633][Streaming] Java API unit test and example for custom streaming receiver in Java

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

Closes #558 from tdas/more-fixes and squashes the following commits:

c0c84e6 [Tathagata Das] Removing extra println()
d8a8cf4 [Tathagata Das] More tweaks to make unit test work in Jenkins.
b7caa98 [Tathagata Das] More tweaks.
d337367 [Tathagata Das] More tweaks
22d6f2d [Tathagata Das] Merge remote-tracking branch 'apache/master' into more-fixes
40a961b [Tathagata Das] Modified java test to reduce flakiness.
9410ca6 [Tathagata Das] Merge remote-tracking branch 'apache/master' into more-fixes
86d9147 [Tathagata Das] scala style fix
2f3d7b1 [Tathagata Das] Added Scala custom receiver example.
d677611 [Tathagata Das] Merge remote-tracking branch 'apache/master' into more-fixes
bec3fc2 [Tathagata Das] Added license.
51d6514 [Tathagata Das] Fixed docs on receiver.
81aafa0 [Tathagata Das] Added Java test for Receiver API, and added JavaCustomReceiver example.