New rule: lines-around-comment

(Dis)allow empty lines around comments. This patch only implements the
lines /after/ a comment line or block. If it is accepted I will adapt it
to consider preceding lines.

The implementation is fully text-based. There is no reliable access to
comments through the official AST API, so I had to resort to manual
parsing. Luckily, JS has some quirks that make it doable.

See https://github.com/eslint/eslint/issues/1344
Looking at this closer, this whole process of marking lines as having a comment seems like overkill to me. Why not just do:

```js
var lineBefore = lines[node.loc.start.line -1 ],
    lineAfter = lines[node.loc.start.line + 1];

if (lineBefore && lineBefore.trim().length) {
      // check
}

if (lineAfter && lineAfter.trim().length) {
      // check
}
```