Support for boolean HTML-attributes

Current `v-attr` implementation doesn't support boolean attributes (e.g. `disabled`).
Here is the common use-case:
```
<button type="submit" v-attr="disabled: !submittionAllowed">
```
It doesn't work because `v-attr` uses `this.el.setAttribute` method. So, when `submittionAllowed` equals to `true`, then submit button will look like this:
```
<button type="submit" disabled="false">
```
...which is treated by browser as disabled button.
My proposal is to remove current attribute if it's value is falsy (except zero).