Do not remove comments from annotated source code.

In my project I need to keep some license comments in the optimized output. I use `uglify` with `preserveComments` set to `some` to achieve this. The problem is that `ngmin` removes all comments before `uglify` has a chance to do anything.

This pull request fixes `ngmin` to keep comments in annotated code. The solution is not ideal as it depends on the experimental API in `esprima` and `escodegen` ([see this issue](https://github.com/Constellation/escodegen/issues/10)), so it's likely to break in the future.

I'm not sure whether it is a good idea to merge this in its current form. In the meantime I have published this fix as new package in npm: [`grunt-ngmin-with-comments`](https://npmjs.org/package/grunt-ngmin-with-comments) and [`ngmin-with-comments`](https://npmjs.org/package/ngmin-with-comments)