Parse Comments

This parses comments and they appear in the output as `{"comment": "comment test"}`  e.g.:

```css
/* http://foo.com/bar/baz.html */

foo { /*/*/
  bar: baz; /* http://foo.com/bar/baz.html */
}
```

becomes

```js
{
  "stylesheet": {
    "rules": [
      {
        "comment": " http://foo.com/bar/baz.html "
      },
      {
        "selectors": [
          "foo"
        ],
        "declarations": [
          {
            "property": "bar",
            "value": "baz"
          }
        ]
      }
    ]
  }
}
```

Note that it ignores any comments that aren't either top-level or direct descendants of `@media` queries.

I'd add support for direct descendants of `@keyframes` but it would break https://github.com/visionmedia/rework/blob/master/lib/visit.js so that needs to be fixed to check `if (keyframe.declarations)` first.

Closes #13