Add support for fields with position: fixed and z-indexes.

This commit alters the adjustPosition() function such that it can
support displaying the Pikaday widget on input fields that are
positioned using position: fixed (either on the field or one of its
parents), and/or fields (or fields with parents) which have a
non-default z-index value.

This is useful e.g. if Pikaday is being used on a field in a dialog
which floats above the rest of the elements on a page.