Honour the important flag

Currently the project ignores all !important instructions during parsing and merges. These commits add support for parsing out the !important flag and honouring that flag during the resultant merge. 

For example, given the following html

```html
<style>div { color: blue !important; }</style>
<div>Default</div>
<div style="color: red">Red</div>
<div class="column-bottom" style="color: yellow !important"> Yellow</div>
```

Premailer.Net will fail to bring the important color styling into the second (Red) div.