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.
The real issue here is making sure xd-shell works in standalone package as well as with combined package. Let's leave Hadoop libs under xd/lib.