Version 2.0.0

Add feature requested in #86 and general clean up and refactoring of API.

• Add new ring paramater to functions didUpdateProgressValue and willDisplayLabel so that they can be used with multiple rings if needed.

Breaking API Changes

• finishedUpdatingProgress(forRing:) now changed to finishedUpdatingProgress(for:)
• didUpdateProgressValue(to:) now changed to didUpdateProgressValue(for:to:)
• willDisplayLabel(label:) now changed to willDisplayLabel(for:_:)
• setProgress(value:animationDuration:completion:) now changed to setProgress(to:duration:completion:)