Banner: delegate banner handling to Uglify's preamble, fixes #191

Lets Uglify do banner handling and consequent normalization independent of the `sourceMap` config. This way, any future issue with the banner should be reported to [Uglify](https://github.com/mishoo/UglifyJS2/issues).

I'm not sure how to write a proper test for this, I believe the ideal unit test would be uglifying the same file with `sourceMap: true` and without, then compare with each other or against the same expected file. Problem is, as each uglify target generates a temp file and checks it against an expected file, I'd need to make 2 copies of the same file with different filenames which is not DRY and does not make the intention of having 2 different settings output the same code output clear -- not very useful seeing as there are already banner tests with and without source maps.

I'd like some tips/suggestions for the tests if these are necessary. 

cc @jmeas