Wrap the content of `modal` in "modal-body"

Before this commit, the following code was valid:

```rhtml
<%= modal 'Content only' %>
```

but resulted in an the following _ugly_ modal:

![ugly](https://cloud.githubusercontent.com/assets/7408595/4958799/806ff02a-66b2-11e4-8997-06f32cb6025b.png)

After this commit, plain-text content passed to `modal` will instead be
wrapped in a `<div class="modal-body">`, and will look like:

![nice](https://cloud.githubusercontent.com/assets/7408595/4958798/806f13ee-66b2-11e4-8b2a-a99613f93205.png)

This is exactly the same as passing the content in the `:body` option:

```rhtml
<%= modal body: 'Content only' %>
```

In case body content and `:body` options are passed, the first takes precedence.