Added no-warning-comments rule (fixes #660)

As mentioned this fixes #660.

* As #580 removed the `getAllComments` function I used the `LineComment` and `BlockComment` AST nodes. The use of them is discouraged in the [documentation](https://github.com/eslint/eslint/blob/master/docs/developer-guide/working-with-rules.md#accessing-comments) but using `getComments` didn't work for this case. Do you know a better way to access all comments?
* My first version had a default warning term `todo` set, to match the [JSLint todo rule](http://jslinterrors.com/unexpected-todo-comment/). I had to remove this because the eslint project itself threw some errors as it has TODO comments on some places. I still think it would be a good idea to enable `TODO` (and maybe even `FIXME`) out of the box. What do you think about it?