Create modals for sign up and join the beta

It is related to #712.

![screen shot 2014-10-13 at 1 42 20 pm](https://cloud.githubusercontent.com/assets/230476/4619896/4aaa2740-531a-11e4-97ff-61be440ff29b.png)

![screen shot 2014-10-13 at 1 42 57 pm](https://cloud.githubusercontent.com/assets/230476/4619897/4aaaaea4-531a-11e4-93be-deb383d22b34.png)


## How to use it?

### To open the modal:
```
$('#sign-up-modal').foundation('reveal', 'open')
```

### Render the html
```
= render 'shared/sign_up_modal'
```
Same thing for the Join the Beta.