Fix multiple declarations of the same property

This simple fix merges properties already declared in the style into a single property as a semicolon-delimited list. 
For example:
```css
.background-gradient {
    background: -webkit-gradient(linear, 0% 0%, 0% 100%, from(#1a82f7), to(#2F2727));
    background: -webkit-linear-gradient(top, #2F2727, #1a82f7); 
    background: -moz-linear-gradient(top, #2F2727, #1a82f7); 
    background: -ms-linear-gradient(top, #2F2727, #1a82f7); 
    background: -o-linear-gradient(top, #2F2727, #1a82f7)
}
```
would result in:
```javascript
cssRules: [
    {
	    selectorText: '.background-gradient',
	    parentRule: null,
	    style: {
	        0: 'background',
	        background: '-webkit-gradient(linear, 0% 0%, 0% 100%, from(#1a82f7), to(#2F2727)); background: -webkit-linear-gradient(top, #2F2727, #1a82f7); background: -moz-linear-gradient(top, #2F2727, #1a82f7); background: -ms-linear-gradient(top, #2F2727, #1a82f7); background: -o-linear-gradient(top, #2F2727, #1a82f7)',
	        length: 1
        }
    }
]
```
This allows the declarations to be preserved as well as correctly parsed, and it will be overwritten if a more specific selector redefines the property. A downside to this implementation is that it does not give individual access to each rule. But since this didn't work to begin with I don't see it as a problem.