plugin interface spec

Verbose version

``` js
var vueTouch = require('vue-touch')
Vue.use(vueTouch)
```

Shorthand equivalent

``` js
Vue.use('vue-touch')
```

Inside plugin...

``` js
exports.install = function (Vue) {
    // Vue.require exposes internal modules
}
```

The `use` function...

``` js
Vue.use = function (plugin) {
    // if string require it
    plugin.install(Vue)
}
```
