getAttributes returns null

feat: always populate MANIFEST with project.name and .version

This pull request directs the plugin to configure the `jar` task (if present, provided by the `java` plugin) and/or the `war` task (if present, provided by the `war` plugin) to set the `Implementation-Version` and `Implementation-Title` attributes in the MANIFEST.MF file included in the result.

The end result is that issue #1 can be satisfied by pulling out `Implementation-Version` from the MANIFEST. By using MANIFEST, we use a file in Java specifications and we don't have to pollute the build of downstream projects with our own custom properties file.

Manifest reference:

https://docs.oracle.com/javase/tutorial/deployment/jar/packageman.html

cc: @paul.erickson @apatwary 




See merge request !2

docs: correct documentation for buildscript block, add local confirmProjectVersionIncrementedTask

Small fix to correct the example provided in the README.

Also includes a way to invoke the confirmProjectVersionIncrementedTask, sharing the logic with the task in the release.

See merge request !1

We don't have a facility to apply the plugin itself without fetching an earlier version.
Includes a script that delegates to the same logic as the task.

Include 'confirmProjectVersionIncremented' task, tests, and basic docs.