Fix for issue #54: input type="submit" should look like pure-button

This pull request targets `.pure-form input` styles to only texty inputs. This fixes the issue of "buttony" inputs such as `submit`, `button`, and `reset` looking different from `.pure-button`.

Here's the issue: https://github.com/yui/pure/issues/54

This PR adds a bunch of rules to `forms.css` but the k-weight gain is minor when gzipped. Here's some k-weight info:

### Original forms.css
```
File build/forms-min.css created.
Original: 8401 bytes.
Minified: 4975 bytes.
Gzipped:  909 bytes.
```

### With this fix
```
File build/forms-min.css created.
Original: 9880 bytes.
Minified: 6336 bytes.
Gzipped:  1050 bytes.
```