Better cross-building and package namespace

Hi! Thanks for the great plugin. It's very useful!

I've just updated sbt-cross-building plugin version and added it's settings to the `build.sbt`. It seems that so far this plugin is not published for sbt-0.13, but I'd like to use it in my plugin based on sbt-0.13.

Also, there is one issue, when I try to use it in my plugin's code: I cannot refer to the stuff in `TagListPlugin` object if it doesn't have an explicit package namespace. So I added it (the same you use in other sources). Change it if you want it to be different.