Converted to annotations, documented most everything.

This also includes a bunch of POM shenanigans to allow the standard "Maven
plugin website" to be generated, which provides documentation for all of the
plugin configuration and which is extremely useful to someone trying to figure
out how to use the plugin.

I've published an example of that website here: http://samskivert.com/robovm-maven-plugin/

You can generate it locally like so:
```
cd plugin
mvn site
open target/site/index.html
```

Ideally this generated site can be published at robovm.github.io/robovm-maven-plugin/. The way that works is that one creates a `gh-pages` branch and everything committed to that branch goes up on this magic website. Once the branch is created, I think I can generate the site and submit a pull request which will ship it. Then I can send another pull request once everything goes to 1.0.

Incidentally, the reason I'm working hard to get the Maven plugin in shape is because the Maven plugin version is tied to the RoboVM version. That's not awesome, but based on the way the code is, it would be hard to change. So I want to be sure that the Maven plugin doesn't suck for the 1.0 release, because I'm going to be foisting it on all the PlayN users who want to use the (forthcoming) new RoboVM backend. :)