Support namespace prefixed arguments

Hi, Evan. This PR is related to #253.
I think the fix 3180eab is not enough in some case.

See: http://jsfiddle.net/DL5gr/4/

This PR enables us to use v-attr with namespace prefixed arguments by escaping a colon.

Example: 
```html
<image v-attr="xlink\:href: url" width="200px" height="200px"></image>
```

I think this solution is very ad-hoc, especially around `isXlink`.
If there is a smarter way, please suggest it.