include build.number in Implementation-Version in manifest

As of the version stated in the Environment field, $

{pom.currentVersion} is used for the both the Specification-Version and the Implementation-Version when creating the manifest for the war.

It'd be nice if the Implementation-Version could include a build number as well, like the one maintained by the Ant buildnumber task.

I have a patch I'll upload that sets the Implementation-Version to "${pom.currentVersion}

, build $

{build.number}

", if build.number is already set.

To use the enhancement, just call ant:buildnumber in a preGoal of war:war.

I can't think of another way to provide this functionality, so I decided to go ahead and patch plugin.jelly. When I started this endeavor, I thought I absolutely had to have this feature, but now that it's done, I could probably live without it. Oh well, that's the way it goes. Hence the minor priority.

Let me know what you think.