Enable selection handler property to be populated via selection-model-handle-events attribute

[avassilev-odbg]

Description:

We need this update in order to trigger selection on different events.
With this change you can specify them in the 'selection-model-handle-events' attribute.
It is evaluated by the selectionModel directive.

Example:

<li data-ng-repeat="child in parent" data-selection-model data-selection-model-handle-events="'click mousedown'" ></li>

Notes:
The attribute value is evaluated by the directive, so you need to specify it in single quotes for strings or provide a getter function.

[jtrussell]

This seems like a useful feature. I'd love to add it but would you mind including some tests? Also, I might be wrong but I think the events should maybe be space separated instead of comma separated?