Use UIAppearance to customize PHRefreshTriggerView

PHRefreshView should support the [UIAppearance protocol](https://developer.apple.com/library/ios/#DOCUMENTATION/UIKit/Reference/UIAppearance_Protocol/Reference/Reference.html). We could then do

```
[[PHRefreshTriggerView appearance] setTitleColor: [UIColor darkGrayColor]];
[[PHRefreshTriggerView appearance] setArrowImage: [UIImage imageNamed:@"betterArrow"];
```

instead of subclassing.
