Added custom classes and resolved issue

[builtbybengt]

added custom classes and resolve issue (#2)

custom classes:

• updated handleDragStart and handleDragEnd to remove / add the classes that were given in options.attributes.draggingClass
• updated handleDragEnter and handleDragLeave to remove / add the classes that were given in options.attributes.dragOverClasses

issue #2:

• created a target element which is the actual element the user is dragging on
• checked if the last element that was entered is the current target element

[Spikeysanju]

@Bxnq, thanks for the PR. LGTM. Merging it!