Make source map truly conditional

Currently source map comments are always present in the minified file because

1. sourceMap options are always set regardless of the value of `sourceMaps`
2. `min.map` always returns true because of this `toString` business going on here: https://github.com/mishoo/UglifyJS2/blob/1a34a13e3393a9afd6a032b5c2bf3e29cbdfe153/tools/node.js#L136

This fixes #12