Redesign visible time logic

This release will change the visible time when weekView first time appears y offset, even if you don't change any code.

You can now set your own visible time in setupCalendar method with variable visibleTime, and the default value for it is current time. If you want something different, you can set this variable by yourself.

You can refer to the CustomViewController in example project.
Also, a new open method can be used to vertically scroll the weekView.

open func scrollCollectionViewTo(time: Date)