Allow Custom Next/Back Images

[MosheBerman]

Summary

[This fork][https://github.com/iChernov/MBCalendarKit/commit/a45db8c3dc7c51ffc3d61ef33a05034439440d43#diff-e4cba162800cbd8cb027cda8d049c5f1R485] swaps out the arrow images. It would be cool to allow these to be configured on the calendar view, and only use the drawn arrows be default.

Steps To Implement

• Add methods to the calendar view to set forwardArrowImageName and backArrowImageName
• Add the properties for the image names to the CKCalendarHeaderView.
• In the header, if those image names are set, load those up.
• If they're not, use the default arrows.

Notes

• There are different ways to do caching of these images.
• Localization is important.