Allow for setting the font via UIAppearance properties.

For example:
```
    UIFont *font = [UIFont systemFontOfSize:12];
    [[MBContactPicker appearance] setFont:font];
    [[MBContactCollectionViewPromptCell appearance] setFont:font];
    [[MBContactCollectionViewEntryCell appearance] setFont:font];
    [[MBContactCollectionViewContactCell appearance] setFont:font];
```