Stops keyboard observation from squashing insets and enables UIAppearance

The first commit fixes an issue where the scroll view's original `contentInset.bottom` and `scrollIndicatorInset.bottom` were overwritten when the keyboard is dismissed.

The second commit cleans up the property definitions to be consistent with the rest of the project and removes unnecessary `NS_AVAILABLE_IOS(...)` macros, since this relies on iOS 7.0 anyway. It also adds support for `UIAppearance`, and sets the defaults previously set in the `-commonInitWithTextStorage:` method. In the example application, I also changed the text-view from using a normal string to an attributed string to verify that the appearance selectors carried through to the attributed text.