Quick hack!
Made numberFormatter a property
Fixed crash when initialized in code not storyboard

Set frame to non-zero size to prevent crash when first calculating
arrowOffset. Offset needs to be calculated before frame so that
‘setValue: animated:’ animates the popUpView correctly.
Calculate _defaultPopUpViewSize

Only recalculate _popUpViewSize when dataSource returns non nil value,
otherwise use _defaultPopUpViewSize.
Updated example with custom string demo
Merge branch 'custom_string'