New signup form

@dennisschoenmakers This pull request adds a new sign-up page to ETModel, fixing both the messed up styles of the current form (#1777) and explaining what the teacher e-mail field means (#1778).

The user will select whether they're a private individual, student, or teacher, and the fields will be adjusted to reflect the choice.

Before being merged, you'll need to add some Dutch translations (I'm not going to trust Google Translate for anymore more than a couple of words). Translations are needed [here](https://github.com/quintel/etmodel/blob/new-signup/config/locales/nl_etm.yml#L223-L232) and [here](https://github.com/quintel/etmodel/blob/new-signup/config/locales/nl_simple_form.yml#L24-L35).

![screen shot 2014-03-10 at 20 26 08](https://f.cloud.github.com/assets/4383/2378774/12e052c6-a893-11e3-96d2-faf2ece30f47.png)

![screen shot 2014-03-10 at 20 26 10](https://f.cloud.github.com/assets/4383/2378781/173dca24-a893-11e3-8310-06ec99df70bf.png)

![screen shot 2014-03-10 at 20 26 13](https://f.cloud.github.com/assets/4383/2378783/18db5b62-a893-11e3-908e-67f9edcdf03d.png)