Styling section views in PM::TableScreen

Implementing issue #211

```ruby
def table_data
  [
    title: "My section title",
    title_view: MyCustomSection,
    title_height: 50,
    cells: []
  ]
end
```

You'll have to set a title accessor on your custom title view class, as the
title you specified in table_data will be set on that attribute.

```ruby
class MyCustomSection < UITableViewCell
  attr_accessor :title
end
```

cc: @jamonholmgren