support two-way data binding (v-model)

[Jinjiang]

Note: Currently the <input> component only support onchange and oninput event but no onkeyxxx event yet. And no API to control position of the cursor when an <input> in focus. It should be discussed with the team later.

[Jinjiang]

https://github.com/jinjiang/vue/tree/v-model
This branch has implemented v-model for <input> and <textarea>. The next step is finishing tests and supporting custom components and extended native components.

[Jinjiang]

And method parseModel() from web/util/model has been moved to compiler/helpers in order to share with both web and weex platforms.

[Jinjiang]

vuejs/vue#4178