Customize notificationDesign

As described in the discussion for [isssue#29](https://github.com/toursprung/TSMessages/issues/29) I added a way to easily customize notificationDesign dictionary, both programmatically and by merging a contents of a user specified json file.

I mistakenly bumped the podspec version number for my own tests, but reverted that change after realizing that should be done by the pod author. Sorry about those extra change lists and thanks for considering the change.