Add support for decrementing for-loops

PR to implement decrementing for-loops as discussed in issue #691 (sorry, couldn't get Hub to attach the pull-request to the issue directly).

From the CHANGELOG:

```
It is now possible to create decrementing loops using the default @for-directive. For example:

    .foo {
      @for $var from 5 through 1 { a: $var }
    }

Produces:

    .foo {
      a: 5;
      a: 4;
      a: 3;
      a: 2;
      a: 1; }
```

I did not add the `by` part to the `@for` loop as suggested by @chriseppstein in issue #691. I'd be happy to implement this in another PR. 

I am a bit unsure about adding the constants to the `ForNode`, they seem a bit out of place there. At first I was going to calculate the direction of the loop in the `ForNode` as well, but you can't do that because the environment can change the values of `from` and `to` later on, right?

Let me know what you think!