fix(bower): add reference to CSS for bower; Fix #502

Tested by using `bower link` to have the `gulp` branch of `encore-ui-template` point to my local copy of `encore-ui`. Here's the partial index.html output of running `gulp wiredep`:

```
        <!-- build:css styles/vendor.css -->
        <!-- bower:css -->
        <link rel="stylesheet" href="bower_components/angular-hotkeys/build/hotkeys.min.css" />
        <link rel="stylesheet" href="bower_components/font-awesome/css/font-awesome.css" />
        <link rel="stylesheet" href="bower_components/encore-ui/encore-ui.css" />
        <!-- endbower -->
        <link rel="stylesheet" href="bower_components/encore-ui/encore-ui.min.css">
        <!-- endbuild -->
```

We'll need to remove the `encore-ui.min.css` reference once this gets merged in and :ship:ed