Users can join a region

https://trello.com/c/W09L9qjM/19-as-a-user-i-can-join-a-region

![screenshot from 2014-11-18 17 11 41](https://cloud.githubusercontent.com/assets/8154429/5097131/005b570c-6f46-11e4-9254-7083c6c4ae20.png)

Perhaps make this conditional on the user not already being in the region? Also, maybe extract to a partial:

``` erb
<%= render "region/join_button", region: @region %>
```

and then in `app/views/regions/join_button.html.erb`:
``` erb
<% if current_user.region != region %>
  <%= form_for current_user do |form| %>
    <%= form.hidden_field :region_id, value: @region.id %>
    <%= form.submit "Join Region!" %>
  <% end %>
<% end %>
```