compression

- changed uglify2 dependency from uglifyjs to uglify-js. The first only
doesn't seems to be maintained anymore (empty git repo, ect.) and
uglify-js also seems to be the right one
- copyright comments will be preserved in js files
- if an xml files contains an <*:pre> tag, the file won't be minified to
preserve the whitespaces (as per html <pre> definition)
- only log warnings if grunt --verbose option is active to reduce noise