Add more specific mouse events that return references to plasmid elements

[rchawdry]

While the ng-mouse events from AngularJS are supported, the mouse events don't provide a reference to the specific plasmid element that was acted upon. This would probably be nice so that when people hover over a marker, you can use the API to get information about a marker (i.e. begin/end) as well call functions on the exposed APIs.