borderRadius getter, specs and consistency

fixes #2545

- fix getter behavior for borderRadius because `border-radius` is buggy in some browsers.(http://jsfiddle.net/9vRwY/)  

For example:

```
# CSS

div {
    border: 5px solid #88A;
    border-radius: 50px;
    height: 200px;
    width: 200px;
}

## JS

element.getStyle('border-radius'); // empty on FF30 and IE11
element.setStyle('border-radius', '20px');
element.getStyle('border-radius'); // now it reads ok == '20px'
```
- make result consistent cross-browser

with this PR `getStyles` always return 4 parameters

- added specs 

With PR code: http://jsfiddle.net/YLQft/
With current code: http://jsfiddle.net/YLQft/1/

__*Note*__: This PR might not pass PhantomJS tests since it uses a old webkit. There is a promise of a new webkit version when they release PhantomJS 2.0. I [tested this on Sauce-Labs](https://travis-ci.org/SergioCrisostomo/mootools-core/builds/27316364) and got green.
I'm wondering if we really need all those prefixed versions. According to http://caniuse.com/border-radius only Firefox 3.x and Safari 4.x would need prefixes, Opera never shipped a stable version using the prefixed property and I'm unsure about Konqueror but quite sure it has been standardized to `border-radius` ages ago.