Allow multiple lines in StringPickerPopover

[nadyapost]

I've found that you can't have multiline strings in the rows.
This pr adds a function to specify a number of lines.

Example:

    let picker = StringPickerPopover(title: "New Picker", choices: ["One", "Two", "Three very long line description that goes to second line or something else"])
    .setSelectedRow(0)
    .setNumberOfLines(0)
    .setDoneButton(action: { (popover, selectedRow, selectedString) in
        print("done row \(selectedRow) \(selectedString)")
    })
    .setCancelButton(action: { (_, _, _) in print("cancel")})