shift click on layers 


![2014-08-15 19_11_56](https://cloud.githubusercontent.com/assets/108094/3940641/c0f71e3e-24ea-11e4-94ff-81114f18758a.gif)

Fix for #49 

1. Shift + click a marker to show that marker + hide all other markers.
2. If any hidden markers exist, show all markers

cc: @ajashton