Use Logger for output

[markslater]

Hi,

I've put together a pull request for switching the plugin output to use a Logger rather than println directly because it gives users of the plugin more control over how much output they see.

The only output I haven't touched is that from VersionDisplayTask because the point of the task is to output to stdout. As it turns out, that actually means I've only changed two classes: the dirty warning in VersioningExtension and various places in SVNInfoService; the Subversion side of the plugin seems to be much more vocal than the Git side.

Personally, I prefer my builds to be mute as far as possible unless something fails, because I find this helps prevent the important output being lost in a sea of text. Other people prefer a more verbose build. The Logger gives us the best of both worlds.

Lastly, I've included a second commit with the non-functional code cleanup my IDE suggested in the files I modified. I don't know what your preferences are on that front, so I wanted to make it easy to exclude those changes if you want to.

[dcoraboeuf]

Available in 2.7.1

[markslater]

Thanks!