Add bh_label helper

This PR implements a helper method for generating a Bootstrap label: http://getbootstrap.com/components/#labels

I didn't realize until after I had finished that [Rails already has a helper method called `label`](http://apidock.com/rails/ActionView/Helpers/FormHelper/label). The weird thing is that it should only be available in the context of `form_for`:

```text
<%= form_for :person do |f| %>
  <%= f.label :name %>
  <%= f.text_field :name %>
<% end %>
```

The alternative, which can be used outside of `form_for` is supposed to be `label_tag`, so I don't know why`label` is bleeding into the global scope.

Anyway, I just decided to prefix the helper name with `bh`. What do you think?