Add a "I represent a company" option to the user form. Fixes #76

I've done this in a very basic way, added some fields to the user and the form.

The label on the check box doesn't render quite as I expected (see image)
![2014-03-24-212231_304x203_scrot](https://f.cloud.github.com/assets/32212/2504827/0d50d72c-b392-11e3-975d-cc19deb7f9b4.png)

Not an expert on simple_form, now I'm doing 

```
= f.input :is_company, as: :boolean, label: "I represent a company"
```

This is even worse:

```
= f.input :is_company, as: :boolean, inline_label: "I represent a company"
```

Any suggestions?