Expose interfaces for CRToast and CRToastView.

In preparation for allowing custom notification views.
Add an option to set a custom view class for notifications.
Make option validator able to validate Class objects.

Use this to tighten up the validation for the notification view class
option.
Merge pull request #2 from mkantor/custom-view

Add option to set a custom view class for notifications.
Merge pull request #1 from Kabam/kabam-master

Kabam master