styling helper that supports composite styles and caching

Summarized in the 'Styling' section in the README.md file. I'm pushing this out for review... it would need tests, and improved documentation, but I wanted to see if this is helpful to others.
This brings up a good point. If we memoize UIImages, the memory usage for PM::FormScreen is going to run off the charts. UIImages have a massive memory footprint, so a simple background image could cause memory bloat beyond what we're comfortable with.

Avoiding this issue won't be easy. Using a lambda that executes "just in time" would be ideal, probably:

```ruby
  def form_data
    [{
      title: "Account Information",
      footer: "Some help text",
      cells: [{
        name: "email",
        title: "ID",
        type: :email,
        value: current_user.email,
        properties: -> {
          {
            "textLabel.backgroundColor" => UIColor.alloc.initWithPatternImage(UIImage.imageNamed("background"))
          }
        }
      }, {
        name: "password",
        title: "New Password",
        type: :password,
        value: ""
      }]
    }]
  end
```

We'd need to cache the lambda and call it "just in time".