Feature/multiselect

[ThibaultJanBeyer]

This solves #2

Adding an independent possibility to add more items and remove items from a previous selection using modifier keys.

Modifiers:

shift, ctrl & command/windows

New property:

** @multiSelectKeys   array           These key will allow the user add more elements to the selection instead of clearing the selection. The only possible values are keys that are provided via the event object. So far: <kbd>ctrlKey</kbd>, <kbd>shiftKey</kbd>, <kbd>metaKey</kbd> and <kbd>altKey</kbd>. Provide an empty array [] if you want to turn off the funcionality. Default: ['ctrlKey', 'shiftKey', 'metaKey']

Other minor changes in 1.6.0:

• renamed class selected to ds-selected
• added a new class ds-hover on elements which are currently covered by the selection node.
• added polyfill for .indexOf for legacy browsers
• added hasClass( @node, @classname ) method