Top inset is now taken into account in certain cases

The top inset (landscape or portrait) is now used to calculate the `top`
when `shouldMoveToTopWhenKeyboardAppears` is set to `YES`. This is only
applicable when the form sheet can fit in the screen’s frame when the keyboard
is visible - otherwise `top` is defined as `0` or `statusBarHeight` so that as
much of the form sheet as possible is visible.
Keyboard movement style is now configured using an enum

Multiple keyboard movement styles are now supported through the use of an enum.
Minor spelling correction, and other refactoring.

Updated examples to use the new enum, rather than the old boolean properties.
Also fixed some broken constraints encountered when running the example on an
iPad.
Fix Issue #59: Top inset is now taken into account in certain cases
Naming refactor