Added :autocomplete option to inputs/input

This adds a helper option to `inputs` and `input` fields to create the correct `autocomplete` html_options.

Based on [this spec][1], if a user passes `:autocomplete => 'billing'` to an `inputs` call, it will set the section (such as billing or shipping) for subsequent autocompletes.

If a user passes `:autocomplete => 'given-name'` to an input field, it will combine that with a previous section to make an input like:
```
<input autocomplete="billing given-name" ...>
```

[1]: http://www.whatwg.org/specs/web-apps/current-work/multipage/association-of-controls-and-forms.html#autofilling-form-controls:-the-autocomplete-attribute