Support strings as property names

This is analogous to the recent 93b23d2c24936d5bd829ba1f725ef442e9475747 (support strings as selectors) commit.

Includes a regression test.

This fixes #36 and should make creating vendor-aware libraries/mixins much easier. An example where this could be useful:

```
.experimental(@property, @value) {
  ~"-webkit-@{property}": @value;
  ~"-moz-@{property}": @value;
  ~"-ms-@{property}": @value;
  ~"-o-@{property}": @value;
  ~"@{property}": @value;
}

.borderRadius(@value) {
  .experimental("border-radius", @value);
}

article {
  .borderRadius(5px);
}

// CSS OUTPUT

article {
  -webkit-border-radius: 5px;
  -moz-border-radius: 5px;
  -ms-border-radius: 5px;
  -o-border-radius: 5px;
  border-radius: 5px;
}
```


Also it should make @metaskills (see [blogpost](http://metaskills.net/2012/02/27/too-less-should-you-be-using-sass/)) happy. :gift: