Show and hide toolbar based on the value of ta-hide-toolbar attribute.

I know there is #42, which includes showing and hiding the toolbar in its feature list, plus many other features. I'm not sure if showing and hiding the toolbar is already supported. I might have missed it. Anyway, here's my implementation of showing and hiding the toolbar based on focus. I'm not sure if what I did is the proper way to do it, but if not, please feel free to comment on my work :)

To use this feature, put the `ta-hide-toolbar` attribute with value `true` inside a `text-angular` directive like so:

```
<text-angular ng-model="model"
              name="model"
              ta-hide-toolbar="true">
</text-angular>
```

PS: My editor stripped out some trailing whitespaces. Please ignore them. Actual change is around line 300.