Fix issue 356 - no interaction with the target element.

Up until now (since it was added in pull request 273), the reference
layer had a higher z-index than the target element, which caused it to
block interaction with target element, as if disableInteraction is
enabled.

To solve this, the reference layer is now hidden, but its children are
made visible. This restores the ability to interact with the underlying
element.

Changing the visibility attribute doesn't affect positioning, so no
other changes were required.
Merge branch 'issue_356_no_interaction' into fixes
Fix issue 369: step number position with floating tooltip

The 'floating' position type was added in be96db029, where it can only
be used when there's no target element. Since there's no visible element
on which to put the number, code was added to position the number layer
next to the tooltip layer in case of 'floating' position.

Since pull requst #273 however, the 'floating' position can also be used
*with* a target element if 'auto' positioning is used (i.e.
_determineAutoPosition can return 'floating'). In this case the code to
position the step number next to the tooltip no longer makes sense, and
it moves the number layer away from the target element.

This commit only positions the step number if there's no target element
(i.e. if the 'introjsFloatingElement' element is used), as initially
intended. If there is a target element, the step number is already
positioned correctly.

Also changed the step number positiion offset from 18 to 16 px to match
the css file.
Merge branch 'issue_369_floating_tooltip_number_position' into fixes