Support new 1.9 hash syntax

Hi,

This change enables to support new Ruby 1.9 style hash syntax like below:

Haml:

```haml
%a{title: 1 + 1, href: "/#{test_obj.method}", height: "50px", width: "50px"}
```

will be converted to Slim:

```slim
a title=(1 + 1) href="/#{test_obj.method}" height="50px" width="50px"
```

Thanks!