Removed MIN_POPUPVIEW_WIDTH & MIN_POPUPVIEW_HEIGHT

- (CGSize)popUpSizeForString:(NSString *)string now calculates size
based purely on the string size and doesn’t enforce a minimum size.
Reduce ARROW_LENGTH to 8.0

Original length (13.0) came from the default in
‘ASValueTrackingSlider’. The size here can be reduced as it doesn’t
need to accommodate the presence of the user’s finger when operating a
UISlider.
Show use of small and large font in example project
Replace blank NSString with default '???'

It’s possible for the dataSource to return an empty string. If this
occurs the drawing code will crash as the size will be CGSizeZero.
Therefore replace empty stings with a default value.
If the user wishes to indicate a blank value they can do so by
returning a tab or ellipsis, or whatever else from the dataSource
method.
Added show and hide as public methods

Both methods take a BOOL argument to decide whether to animate or not.
The property ‘alwaysShowPopUpView’ has been removed. This means that
the caller is now always responsible for showing and hiding the
popUpView. The popUpView will no longer automatically display when the
progress is set to a value > 0.0.
Removed _popUpViewIsVisible ivar

Check the alpha value of self.popUpView rather than use an extra BOOL
ivar to track visibility of view.
Changed timer update frequency
Code cull: do more with less!

Make frame adjustments to popUpView in ‘layoutSubviews.’
Update color animation offset in ‘setProgress:’
Remove KVO for ‘progress’ and ‘bounds’
Remove custom animation method in popUpView
Add ‘actionForLayer:ForKey:’ to set-up animation
Add ‘animateBlock:’ to popUpView to animate any view updates
Adjusted CATextLayer for iOS8

NSAttributedString NSForegroundColorAttributeName must take a UIColor
in iOS8, but this causes issues on iOS7 when used with CATextLayer.
Therefore set foregroundColor property directly rather than with
attributedString.
No need to set frame of _pathLayer in layoutSubviews

_pathLayer is the main layer of the view, it will be automatically
adjusted when the view frame is set.
Remove size checking in layoutSubviews

The CAShapeLayer path is no longer redrawn in layoutSubviews, so
there’s little benefit to the check.
Conditionally redraw path when setting frame

Only redraw path if popUpView size or arrowOffset have changed.
Refactored popUpView updating

The popUpView is now only updated using current values, so the more
flexible block method approach is no longer needed.
Combine if clauses into one
Renamed local variable
Added comments
Reduce default font size
Merge branch 'popup_size'