Support the view controller be embedded in container view

Calculate the bottom margin to its enclosing window. This unifies `appropriateTabBarHeight` consideration.
Add container view controller support to README
Merge pull request #73 from ikesyo/container-view-support

Supports the view controller be embedded in a container view (ie: UITabBarController or any other custom container)
Removes (unused) obj runtime import
Improves external keyboard assumption & avoid resigning first responder on esc press key on external keyboard
Re-enforces gesture recogniser acceptance. This was causing an issue with external keyboard not allowing tableview cells to be interactive.
Minor tweak
Makes sure `scrollViewProxy` is always set. Should fix #75
The property `shouldScrollToBottomAfterKeyboardShows` was never being checked. Fixes #72. Thanks @CraigSiemens for reporting this.
Simplifies UIScrollView additions. Improves scrolling APIs.
Minor refactoring regarding auto-completion. Makes sure reloadData is only called when attempting to show the auto-completion view.
Exposes `-didPressArrowKey:` to be able to perform additional tasks. Also fixes up/down arrow detection being registered twice, and colliding, causing not to be called in SLKTextViewController anymore.
Makes changes to the sample project, based on the latest key command API improvements.
Makes undoing an opt-in feature. Default is Yes.
Clears the undo manager after pressing the right button
Clears the undo manager when resigning first responder. Also, fixes the undo manager displaying undo/redo options when there was nothing to undo/redo.
Updates right button style