Shorthands - closes #29

I had to change the implementation of CSSRuleSet.
The rules list is now a numerically indexed array instead of an associative array.
This allows us to have things like this:

```css
p {
  border-style: solid !important;
  foobar: baz;
  border-style: none;
}
```
And then do this:

```php
foreach($oRuleSet->getRules('border-style') as $position => $rule) {
  printf("%d => %s", $position, $rule);
}
/*
0 =>  border-style: solid !important;
2 => border-style: none;
*/
echo $oRuleSet->getAppliedRule('border-style');
/*
border-style: solid !important;
*/
```
This allows expand/createShorthands to get around some edge cases.