Change / fix for: Preserve sub-directory structure when only minifying

This patch mainly addresses https://github.com/samaxes/minify-maven-plugin/issues/29 . A slight change of the way file extensions are handled also allows for a bit more freedom in suffix specification. Previously the dot from the extension was prepended to the suffix due to how FileUtils created the base name - now that dot is optional, added only if the suffix specifies it. The default suffix has been changed to keep the current default behavior.