add a price_field method

Assuming we're in a `twitter_bootstrap_form_for` block with `user` as the block variable:

the attached code makes this:
```
= user.price_field :balance
```
equivalent to this:
```
= user.text_field :balance, :add_on => :prepend do
  %span.add-on= t('number.currency.format.unit').html_safe
```