Add objects support to v-attr

[OEvgeny]

The goal is to add to v-attr directive support for objects:

<div v-attr="obj"></div>

Demo here: http://jsfiddle.net/nd9baygn/3/

Waiting for suggestions patiently :)

[yyx990803]

Looking good! Let's just make things consistent to resemble the updated v-style implementation: https://github.com/yyx990803/vue/blob/dev/src/directives/style.js

I think the logic should be equivalent and I don't see the need to store the previous object as this.obj.

[OEvgeny]

Done!

[yyx990803]

good job!