Evil Studios changes + Cocoapods support!

This PR refactors most of the internals of `MPGNotification`, and some of the API. Very minor (2-3 pixel) adjustments to some UI elements to provide more padding between visual elements.

The goal of the internal code refactor is make the project more extensible, and allow customizations and new properties that were not possible before. The changes are too numerous to list, but please check the diff for more info.

The goal of the external API refactor is to provide a more modern interface with which to call the class. In particular, Apple has generally abandoned long `init`-type methods in its new classes in favor of class method convenience constructors. (This seemingly small change actually created the majority of the code work in this PR.)

Read the README in this PR for details on the new API. It's also available on the web (and formatted with Markdown/much easier to read), here:

https://github.com/evilstudios/MPGNotification/tree/return-home