XD-2148 add distShellZip gradle task

With this change gradle dist task now creates spring-xd-shell zip distro under build/distributions. 
The spring-xd-shell distro size is rather large - 144MB. 
Next in line is XD-2149 to remove unused libraries.
Its xd/lib since xd-shell is doing this:

XD_LIB=$APP_HOME/../xd/lib
HADOOP_LIB=$XD_LIB/$HADOOP_DISTRO
addJarsToClassPath $HADOOP_LIB