Add button_html and button_class options for "flash_messages"

useful when templates has `x` better styled, e.g.


``` slim
= flash_messages
```
![screen shot 2014-11-05 at 12 31 11 pm](https://cloud.githubusercontent.com/assets/473/4912816/9b2b3d12-64a4-11e4-8e88-d78420b2b253.png)


``` slim
= flash_messages(button_class: 'close fui-cross', button_html: '')
```
![screen shot 2014-11-05 at 12 30 23 pm](https://cloud.githubusercontent.com/assets/473/4912812/835da080-64a4-11e4-9cc2-d43167f3cc83.png)