Incrementing gloabl z-index can cause issues

[brian-clariture]

Incrementing the global z-index value (

opentip/lib/opentip.js

Line 507 in 4913cbf

zIndex: Opentip.lastZIndex++

) was causing some problems for us. We load a lot of tips and the z-indices were getting out of control. I'd propose at the very least making this optional. For our use case, which I don't think is uncommon, a single z-index for all tips is preferred.