Use /*! … */ comment format for banner so that external minifiers know to preserve it

KO ships its own pre-minified file, but nonetheless sometimes people will use their own minifier, either on the `.debug.js` file or even on the already-minified `knockout-x.y.z.js` file (perhaps because they are bundling it with other files, and then they run a minifier over the whole bundle). The most common one is uglify.js.

Currently, Uglify strips KO's license header. This pull request uses the special `/*! ... */` comment format for the license header, so that Uglify's `preserveComments: 'some'` option knows it should retain it.

Most JavaScript libraries use this format for license headers - I don't see any reason for KO not to :)