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

Before this commit, the following code was valid:

```rhtml
<%= panel 'Content' %>
```

but resulted in an the following _ugly_ panel:

![ugly](https://cloud.githubusercontent.com/assets/7408595/4946816/afa9dc8c-6619-11e4-897c-0cc2ecc39a7a.png)

After this commit, plain-text content passed to `panel` will instead be
wrapped in a `<div class="panel-body">`, and will look like:

![nice](https://cloud.githubusercontent.com/assets/7408595/4946817/afaa1d0a-6619-11e4-9899-2b0a5cc9a378.png)

This is exactly the same as passing the content in the `:body` option:

```rhtml
<%= panel body: 'Content' %>
```

In case body content and `:body` options are passed, the first takes precedence.