Added a way to change placeholder's color of QEntryElement.

Many designers don't like the default color :)

Now, you can change it just like text color.

```
-(void)cell:(UITableViewCell *)cell willAppearForElement:(QElement *)element atIndexPath:(NSIndexPath *)indexPath {
    QEntryTableViewCell *entryCell = (QEntryTableViewCell*)cell;
    [entryCell.textField setTextColor:[UIColor blackColor]];
    [entryCell.textField setPlaceholderColor:[UIColor blueColor]];
}
```