Add initWithCoder: initialization code path

This pull request adds an initialization code path via `initWithCoder:` for use when the scroll bar has been added to the view hierarchy via a XIB/Storyboard.

There is a secondary commit that adds the Cocoapod podspec to the project. It is currently necessary to have a podspec in the project to enable installation via Cocoapods from a fork/branch by overriding the :git and :branch options.