Added ability to add UIView or UIBezierPath as a placeholder before progress starts.

The default arrow bezier path in control is not always the desired
icon. This adds the ability to render an UIView or UIBezierPath if set.
Otherwise the default arrow is rendered in it’s place.
Merge pull request #9 from Legoless/master

Added ability to add UIView or UIBezierPath as a placeholder before prog...