Put a whitespace before the inline_label if it is present

To align with the Bootstrap interface we need to put an space before the inline label:

![screen shot 2014-03-28 at 18 43 30](https://cloud.githubusercontent.com/assets/47848/2555613/1923fd62-b6c2-11e3-88b7-188f96437478.png)

So when we call:

```erb
<%= f.input :checkbox, as: :boolean, inline_label: 'Remember me' %>
```

The label will be placed correctly.

I'm not sure if we should do this by default or require the users to put the whitespace in the label text like this:

```erb
<%= f.input :checkbox, as: :boolean, inline_label: ' Remember me' %>
```

cc @carlosantoniodasilva