Extend the application plugin to build a tar distribution

I implemented the community contribution idea posted here:

http://forums.gradle.org/gradle/topics/extend_the_application_plugin_to_build_a_tar_distribution

I added a distTar target to the application plugin, in the same manner as distZip.  I haven't implemented the "extra credit" yet, but I'll work on it shortly.

I would love any kind of feedback, as this is my first attempt at contributing.