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.