Add zIndex support for marker

[paabrown]

1. Allow user to pass 'zIndex' and 'optimized' props to marker class. Setting the 'optimized' property to 'false' seems to help the user determine which marker displays on top using the zIndex property. See: https://stackoverflow.com/questions/11845916/google-maps-marker-zindex-doesnt-work-for-two-icon-types-symbol-and-string

[blainegarrett]

I'm just making my way around the lib, but it seems like maybe this merge caused a "conflict" in the dist package?
https://github.com/blainegarrett/google-maps-react/blame/master/dist/components/Marker.js

I'm seeing the following in my package which is causing syntax errors:

var _props = this.props,
  ...    
  title = _props.title,
  animation = _props.animation;
  optimized = _props.optimized,
  zIndex = _props.zIndex;

[paabrown]

You're correct - this should be fixed