XML comments not highlighting correctly
If I have some XML in a code block and put a comment after the tag, the comment isn't correctly syntax highlighted. Example:

``` xml
<element attribute="value" /> <!-- attribute = value -->
```

It seems to correctly color it (light and italics) if I put a `//` inside the comment:

``` xml
<element attribute="value" /> <!-- // attribute = value -->
```

Result:
![39](https://cloud.githubusercontent.com/assets/94210/2589904/9ff50086-ba53-11e3-9bbb-e5a8c50cc265.png)
