Changed zIndex to auto rather than initial

Attempting to use initial as the zIndex on IE8 causes an error, which
stopped this working altogether with IE8.

Accoring to MDN the only valid values for this attribute are `auto` and
an integer: https://developer.mozilla.org/en-US/docs/Web/CSS/z-index
Merge pull request #28 from obmarg/zindex

changed zIndex to auto rather than initial