XD-36 HDFS outbound adapter initial commit

ignoring integration test since it requires Hadoop

we should add a JUnit rule to check whether Hadoop is running
sink/hdfs.xml uses namespace
XD-27,XD-65 added spring-xd-gemfire-server module
XD-73: add test to start/stop stream server
Merge pull request #6 from wkoh/servertest2

XD-73: add test to start/stop stream server