Add support for resource as <item type=string ...

While testing an activity using SmoothProgressBar from @castorflex (due to ActionBar-PullToRefresh), I had problems due to this resource:
```    <item name="spb_default_speed" format="float" type="string">1.0</item> ```

This bug has already been tracked on the Google groups at https://groups.google.com/forum/#!topic/robolectric/raKiC51lLzM

This PR only adds support for this kind of resources.