Added new Custom alert view style enum.
Added Custom enum case to switch-statement that sets the image and color passed in, as well as grows the icon image view to be larger inside the circle view.
Added image and color params to the default showTitle method so they could be passed in for the custom alert view type, and are left nil for all others.
Added git icon for demoing the custom alert style.
Exposed the new showCustom method in the header.
Updated the storyboard with a new buttom to demo the custom feature, and added the outlets to handle creating a new custom alert and displaying it.
Added documentation for the new showCustom method.
Updated README with new custom info.
Added screenshot image for custom.
Merge branch 'custom-icon'