Set correct initial zIndex on tile layer divs

Fixes #1683.
Merge pull request #1683 from jfirebaugh/1683

tile layers added after initial load don't show up until zoomed
calculate x position with postion:relative and width/maxWidth
smart tabs per contribution guidelines
Merge pull request #1684 from scooterw/1670

calculate x position with postion:relative and width/maxWidth
Check object when LatLngBounds extend

When LatLngBounds extend, must check the extending object, because empty marker `L.marker().getLatLng()` got `undefined` and `obj[0]` got `TypeError`
Update LatLngBoundsSpec.js
Merge pull request #1688 from korzhyk/master

TypeError prevention on LatLngBounds extend
add mailmap to gitignore