Clicking and dragging on a thang from Add Thangs allows you to place it on the map

Ah, one of these after so long.

Basically, most of the work being done in mousedown handlers is being done in mouseup handlers instead, and @selectAddThang is triggered on mousedown instead of a mouse click.

Quick question about this line - 
```
$('.tab-content').click @selectAddThang
```
The selector covers pretty much the entire page, seems to be used only for the add thangs column on the right though. If not, there might be a minor issue with my code.

Should take care of #170