make map div id unique

fix for #5 

Did think about simply adding `item.id` to the map's div id, but that would not be enough as view either a previous version, or another copy, of the page would try and create maps with none unique names.

So, just create a [unique id](http://coffeescriptcookbook.com/chapters/strings/generating-a-unique-id) and append that to 'map'.