Refactor, Update, and Subspecs

This pull request (which closes #10) does the following:

* Refactors the existing code to improve maintainability and adherence to Apple's guidelines regarding property access.
* Makes the `backdropView` optional via a property on `BSModalPickerBase`.
* Fully updates the library to be iOS 5.x+ exclusive.
* Updates the `README` documentation.
* Adds `subspec`s to the `podspec` so that it isn't necessary to pull the whole library when using CocoaPods.